Its been a month & this question hasn't received a good enough, acceptable answer.
Shortly after posting the question here, I posted it on Quora as well. Only difference is that, there I asked people to speculate & be imaginative. One person named Steve Jones came up with a brilliant answer with a thorough, in-universe explanation to the what if events.
Below, I present his answer in unedited form
Let us assume that the events of Deathly Hallows takes place exactly
as in the book up until the time of Snape's death, with one key
exception:
"It cannot be any other way," said Voldemort. "I must master the wand, Severus.
"Master the wand, and I master Potter at last."
Voldemort raised the Elder Wand, pointed it at Snape and hissed "Avada Kedavra."
With a burst of green light, Severus Snape hit the floor.
"I regret it," said Voldemort coldly.
He turned away; there was no sadness in him, no remorse. It was time to leave this shack
and take charge, with a wand that would now do his full bidding.
Return to Hogwarts
At this point, Harry would likely still have gone
to Snape's side, and after Voldemort's speech would have returned to
the castle with Ron and Hermione, but importantly without the memories
from Snape. During the walk back, Harry is haunted by Voldemort's
words
"You have permitted your friends to die for you rather than face me yourself.
I shall wait for one hour in the Forbidden Forest ... one hour ..."
When Harry returns to the Great Hall and sees the Weasleys crowded
around Fred's body, and then catches sight of Lupin and Tonks, he is
so distressed that he turns away and runs up the marble staircase to
the Headmaster's office, to look at Snape's memories. However neither
Dumbledore nor any of the other portraits are there. In our
"alternate" version, Harry might still go up to the office to speak to
Dumbledore's portrait.
"Password?"
"Dumbledore!" said Harry without thinking, because it was he whom he yearned to see,
and to his surprise the gargoyle slid aside, revealing the spiral staircase behind.
Of course the office would hold no answers for Harry, so we must now
infer what he might do next from his previous actions.
The Forest
In Philosopher's Stone, he told Hermione to go back for Ron
while he went on alone. When Sirius and Dumbledore died, he left
everyone else behind and chased their killers through the Ministry and
Hogwarts respectively. Coupled with the guilt he feels that his
friends have died because of him leads me to believe that he would
still have put on the Invisibility Cloak and gone down alone into the
forest, to try to kill Nagini and then Voldemort, but not knowing that
he housed a fragment of Voldemort's soul which also needed to be
destroyed. As with most of his attacks in the rest of the book
(against the Death Eaters in the cafe in Tottenham Court Road, against
Umbridge in the Ministry, against the goblins at Gringotts; the
principle exception I suppose is Malfoy Manor) I believe he would have
tried to use stealth to achieve victory. On the way, he would still
have told Neville about Nagini as a back up plan, but I don't think he
would necessarily have opened the Snitch or used the Resurrection
Stone in the same way.
However as soon as he fired a curse at Nagini in her protective cage,
he would have been found. He may have managed to take down a few Death
Eaters on the way, but Voldemort would have been too vigilant to have
taken too great a hit.
"I thought he would come," said Voldemort in his high, clear voice, his eyes on the
leaping flames. "I expected him to come." ...
"I was, it seems ... mistaken," said Voldemort.
"You weren't"
Harry's Death and Resurrection
In the book, Harry willingly allows
Voldemort to cast the Killing Curse against him. In our alternate
version, Voldemort may have had a more aggressive Harry. He would have
been surrounded by Death Eaters, like in the graveyard of Little
Hangleton at the climax of Goblet of Fire, and one of three scenarios
might have then happened:
- Harry simply allows Voldemort to kill him (like in the book Deathly Hallows)
- Harry and Voldemort duel but the Death Eaters interfere. Whilst they have been instructed not to kill Harry, they may well have tried
to disarm or incapacitate Harry, and allow Voldemort to humiliate and
kill the defenceless boy.
- Harry and Voldemort duel with no interference from the onlooking Death Eaters (like in the book Goblet of Fire).
In scenario 1 and 2, just like in the book, Harry is protected by his
mother's sacrifice living on in Voldemort's body, and also his being
Master of the Elder Wand. Harry might therefore visit "King's Cross"
(like in the book) and Dumbledore would then tell him the truth about
Voldemort's soul and Snape's allegiance and send him back to the
living world. Hence the plot from the book is followed pretty closely.
However in scenario 2 there are 2 key differences from the book.
Firstly, if someone has managed to disarm Harry, then he might no
longer have been Master of the Elder Wand, but he may have been able
to recapture his wand in the fracas outside Hogwarts (when Neville
kills Nagini). Secondly, since Harry did not willingly give up his
life to save his friends, he would not have given them magical
protection. This might have made Neville breaking free from the Body
Bind Curse to kill Nagini difficult, but someone else (or even Harry)
could have removed the spell from him. Some of the other allies who
fought when battle resumed may have sustained greater injuries.
Scenario 3 -
Voldemort defeated but not killed In scenario 3, Harry
would likely have overpowered Voldemort in the Forbidden Forest, just
like he did in the Great Hall in the book, as Harry was still Master
of the Elder Wand. However Voldemort would not have died as he still
had surviving Horcruxes (Nagini and Harry).
This then leads to 2 possibilities. Either the Death Eaters (who still
surround Harry) would have killed Harry (which eliminates another
Horcrux, however I do not think given this circumstance that Harry
would go to "King's Cross" and return, so you would need another means
of revealing the true nature of the scar connection), or they don't
kill Harry (perhaps they flee, perhaps they surrender, perhaps they
simply let him go and they regroup to continue attacking the school,
perhaps they tie him up to parade him in front of the school). We do
know however that the remaining inhabitants of Hogwarts, who are soon
joined by reinforcements from Hogsmeade and the Centaurs and House
Elves are able to defeat the Death Eaters if it came to a battle as
this happens in the book.
Voldemort's remaining essence would likely have possessed Nagini's
body and fled, just like he fled after his first defeat by Harry as a
baby. Ron, Hermione and Harry (if he survived) would know that Nagini
needed to be killed before Voldemort; Neville would know that Nagini
needed to be killed but not why nor any more detail than that. And if
he survived, Harry would still unwittingly carry part of Voldemort's
soul.
This then would have to set up an 8th book in the series, where the
heroes try to track down Voldemort and Nagini, Voldemort tries to
regain a body again and maybe experiments with making more Horcruxes,
Hogwarts and the Ministry have to rebuild and restore order to the
Wizarding World, and the Death Eaters have to decide whether they can
continue to follow Voldemort having witnessed his defeat to a child
for the second time, this time in person (ok technically Harry is an
adult the second time).
Here is the link to his answer on Quora
If you found his answer as brilliant as I had, please upvote it on Quora
