Well, this has been sticking in my craw for a while, so I better get it out.
This question is based in the premise that Ilúvatar "tripped" Gollum at the Cracks of Doom, making it "inconsistent" not to trip Isildur the last time the Ring was there. But
Ilúvatar didn't trip Gollum.
My heart tells me that he has some part to play yet, for good or ill, before the end; and when that comes, the pity of Bilbo may rule the fate of many -- yours not least.
-Gandalf to Frodo, Book I, chapter 2, "The Shadow of the Past".
"Making Gollum trip" is a common, but incorrect, over-interpretation of this passage in Tolkien's letter (#192) to Amy Ronald on 27 July 1956:
Frodo deserved honour because he spent every drop of his power. The Other Power then took over: The Writer of the Story (by which I do not mean myself), the one Person who is never absent and never named.
But this is what Tolkien actually wrote in the book:
And with that, even as his eyes were lifted up to gloat on his prize, he stepped too far, toppled, wavered for a moment on the brink, and then with a shriek he fell.
-Book VI, chapter 3, "Mount Doom"
Gollum, in his elation over having finally gotten the Ring back, wasn't paying attention to his surroundings and stepped over the edge.
We do have these interesting interactions with Gollum in Book IV:
"Sméagol will swear on the Precious".
Frodo drew himself up, and again Sam was startled by his words and his stern voice. "On the Precious? How dare you?" he said. "Think"!
One Ring to rule them all and in the darkness bind them.
Will you commit your promise to that, Sméagol? It will hold you. But it is more treacherous than you are. It may twist your words. Beware!"
-Frodo and Gollum, Bool IV, Chapter 1, "The Taming of Sméagol"
Later:
If I, wearing it, were to command you, you would obey, even to leap from a precipice or cast yourself into the fire. And such would be my command.
-Frodo to Gollum, Book 4, Chapter 3, "The Black Gate is Closed"
And finally, after Gollum's attempt to get the Ring on the slopes of Mount Doom:
Then suddenly, as before under the eaves of the Emyn Muil, Sam saw these two rivals with other vision. A crouching shape, scarcely more than the shadow of a living thing, a creature now wholly ruined and defeated, yet filled with a hideous lust and rage, and before it stood stern, untouchable now by pity, a figure robed in white: but at its breast it held a wheel of fire. Out of the fire there spoke a commanding voice:
"Begone and trouble me no more! If you touch me ever again, you shall be cast yourself into the Fire of Doom."
-Book VI, Chapter 3, "Mount Doom"
The first two are possibly mere foreshadowing. The third is a little more important. First, this is apparently the point where Frodo was pushed over the edge into wielding the Ring. Second, it appears the Ring itself, not Frodo, is warning Gollum what would happen if he touched it again. And this is exactly what happened.
Letter 192 was written in response to a comment that Frodo was somehow "bad" for finally giving in and claiming the Ring for himself. But Frodo had spent all of his strength just getting to the Cracks of Doom. The only way the ring was going to go into the Fire was from someone who craved it just as much as Frodo did, and was heedless of the consequences.
So, how and where did the Maker of the Story intervene? It was all the way back in the Hobbit, when pity stayed Bilbo's hand from slaying Gollum.
This excellent Youtube video by In Deep Geek helped focus my thoughts.