There are three critical components to consider.
One: There is the difficulty of using portkeys; portkeys are not supposed to be used unless prior authorization by the ministry has occurred.
Although it seems Dumbledore can easily break this rule;
"Now see here Dumbledore!" said Fudge, as Dumbledore walked over and
picked up the golden head "you haven't got authorization for that
portkey! You can't do things like that in front of the Minister of
Order of the Phoenix Chapter 36 (pg. 818 in the
Scholastic edition from 2003)
As far as we are aware, Dumbledore is the only wizard to break this rule and although he does so easily, there is another example in Order to prove breaking this rule is no easy task.
"How're we getting - wherever we're going?" Harry asked. "Brooms,"
said Lupin. "Only way. You're too young to Apparate, they'll be
watching the Floo Network, and it's more than our life's worth to set
up an unauthorized Portkey."
Order of the Phoenix, The Advance Guard, Pate 52 (Scholastic:2003)
In the example above, there were a number of highly capable wizards present, Lupin, Moody, Shacklebolt, . . . I'm going to say Crouch probably wasn't capable of it either, prior to training from Lord V and even then, it may have helped him in the measure of success he did obtain in that the trophy was already a portkey.
The Triwizard Cup was an "allowed" portkey that was authorized and was supposed to transport the winner out of the maze and back to the stands to be celebrated. Even if Crouch Jr. thought he had turned it into a portkey, in reality, he only altered it a little before placing it in the maze by adding an extra portus to the graveyard. Perhaps, this made Crouche Jr's job easier because there was no need for spells to make it difficult to detect an unauthorized key, perhaps it doesn't really matter in the end in terms of how difficult his job was and he learned what he needed from Lord V first, either way it was not, in fact, an unauthorized and brand-new made from scratch portkey, but already had the portus to the grandstands on it when Barty did his bit.
Two: Voldemort felt he needed to put on a show for his not-so-satisfactory-Death-Eaters (the ones not in Azkaban) to prove to them he was as powerful as ever.
While Crouch could have killed Harry and then escaped, Voldemort wanted to kill Harry himself and he needed Harry's blood. It hurt Voldemort's pride that he had been defeated by a baby and he felt he needed to prove there wasn't really anything special about, "the boy who lived." He talks about this need in "The Death Eaters" when he says:
"You know of course, they have called this boy my downfall. . . but no
matter, I can touch him now! [Harry feels terrible pain in the next
paragraph while V touches him]
he goes on, but you get the idea.
It is quite clear that Lord V understood he couldn't touch Harry because of what occurred in the chamber at the end of Sorcerer's Stone. Lord V needed Harry so he could have Harry's blood. Simply having Crouch kill him would not have worked because although it would get rid of V's need to touch the boy, it wouldn't get rid of his need to prove Harry was not his undoing or stronger than he.
Just before having Harry released for the Duel Voldemort says:
"You see, I think, how foolish it was to suppose that this boy could ever have been stronger than me, but I want there to be no mistake in anyone's mind. Harry Potter escaped by lucky chance and now I'm going to prove my power by killing him here and now. . ." Emphasis mine.
Three: Harry needed to be ported to Voldemort at an expected time and a particular place. I'm sure there was a complicated bit of spell work and a potion involved here. Who knows how long it took for the potion to be ready for adding Harry's blood in the first place. Keeping Harry captive for any length of time would likely have proved dangerous as it surely would have brought Dumbledore looking. One of the first things Dumbledore would likely have done once Harry had been identified as missing, would have been to check out any location connected to Voldemort and his past anyway.
Even though we don't know for sure they had to wait that long for the potion to be ready, We do know throughout the book, Voldemort is getting progressively stronger (Dark marks of Karkaroff and Snape as evidence). I'm sure he wanted to ensure sufficient strength for the spell to work and to look good for his cronies. So just having Harry show up whenever Crouch could happen to get him to touch the proper pen where no one else would see and at a time when he was not expected at another class or whatever would also have been risky.
Just for the sake of argument, let's say Voldemort felt he could keep Harry captive long enough for V's strength to be what it needed and the potion to be ready, how would he be sure to capture Harry if Harry arrived at an unexpected time? Until he was able to call the other deatheaters to him, Voldemort only had Pettigrew, Nagini and Crouch. Pettigrew was busy with caring for and strengthening V, and Crouch was busy with getting Harry to the set location. As we know, there wasn't anyone there to intercept Harry right away when he arrived in a short window of expectation. Had he arrived at the graveyard with a window of "any time that year" there would have been a chance he could have walked right out of the graveyard or some such other "escape." because no one was there to make sure he was captured. I don't think Voldemort trusted the other Deatheaters to let Pettigrew go through with it, he certainly didn't want all of them seeing him in his weakened state.
Finally, Voldemort intended that it would Appear as though Harry simply died/inexplicably went missing in the maze
It is mentioned in both the book and the movie that contestants have died battling in the Tournament in the past - this is partly why the age limit is added - to protect the younger witches and wizards that would be most likely to have the inability to protect themselves from a permanent injury or fatality.
Voldemort did not want anyone to know yet of his return, so he planned on using the history of occasional deaths during the tournament to his advantage. A death during any other part of the year would result in much stronger inquiry and much deeper searching into a cause. He hoped the tournament would provide a cover to explain Harry's death without too much questioning.
He still needed to be sure he was at his own full strength (including having all of his best supporters by his side - which meant breaking them out of Azkaban) He needed secrecy in order to easily free those Death Eaters still in Azkaban and to infiltrate the ministry sufficiently to prevent organized resistance on a government level.
Voldemort mentions to his returned DE's about gaining the allegiance of the dementors next and getting those "entombed in Azkaban" back and even goes on to mention getting the allegiance of the giants as well. While he doesn't explicitly identify an order of occurrences, within the context, it does not sound at all like a plan to reveal himself back at Hogwart's right away (650-651).
Additionally, Barty Crouch/Mad-eye Moody clearly expects Lord V to torture if not kill all of those who were able to go to the graveyard that night because they were neither locked up, nor dead. Certainly, he would not be under that impression if they planned to show up at Hogwart's ready for a fight after killing Harry (674-675).
The real kicker for me to refuting the idea that it was made into a return portkey by Voldemort/Crouch, is that Dumbledore points out,
"Voldemort has returned," Dumbledore repeated, "If you accept that
fact straightaway, Fudge, and take the necessary measures, we may
still be able to save the situation. The first and most essential
step is to remove Azkaban from the control of the dementors" . . .
Fudge reacts negatively . . . "The rest of us sleep less soundly in
our beds, Cornelius, knowing that you have put Lord Voldemort's most
dangerous supporters in the care of creatures who will join him the
instant he asks them!" said Dumbledore. "They will not remain loyal
to you, Fudge! Voldemort can offer them much more scope for their
powers and their pleasures than you can! With the dementors behind
him, and his old supporters returned to him, you will be hard pressed
to stop him regaining the sort of power he had thirteen years ago!"
Emphasis is mine - pg 707.
The reason this is a key ingredient in this discussion is that it shows clearly, Lord V still had work to do before the likes of Dumbledore would consider him a truly serious threat - and that it was work that needed doing before the ministry knew of V's rebirth. Showing up at Hogwart's triumphantly having killed Harry was most definitely not V's plan here. He is pompous and proud enough, showman enough to want that, but also way too clever to do it and let his secret out way too soon for ultimate success. He still fears Dumbledore and doesn't ever seek out having to kill Dumbledore himself because of that fear.
For these reasons I am convinced the "return" aspect of the portkey is placed there by Dumbledore (or someone else he trusted such as Minerva) before he ever hands it over to Barty to take into the maze. It is placed there as a convenient way to get the winner of the tournament to the stands and out of the maze, as well as a sure-fire way to know who the winner was in case of a close race. Barty and Lord V would have considered such a prior portus irrelevant and of no concern because it simply, would never get used (in their thinking). Or, they may have even considered it advantageous as it made it easier to add the portus charm to the graveyard without discovery. They would never have considered the possibility of Harry's escape from Voldemort in the first place so they certainly would not have bothered with trying to remove it.
This leaves only one hole I can think of
How did Voldemort figure on explaining the disappearance of the trophy? My conjecture is that he didn't. Voldemort does have a history of overlooking plenty of details throughout the books. The love of mothers for their sons is only one of these details.
And I'm not just speaking about Lilly's love for Harry. Narcissa's love for Draco is included here. Then there is also hiding the diadem right at Hogwart's in a room he somehow supposes others won't find. Or what about Kreature's role with the locket and the idea of Regulus defying him? . . .
The list of overlooked details on the part of Voldemort could be quite long if I really got into it. The point is that it is completely feasible that Voldemort would overlook such a detail figuring the ministry might figure something had also happened to it when Harry also disappeared. Or, Rowling overlooked that detail herself or both.