Just to say we don’t know what Gandalf’s plan was - as it is never explained. He would not have sent them via Cirith Ungol, with the watchers and the giant spider, and going past Minas Morgul - as he reacted to that news with horror.

Gandalf put his hand on Pippin’s head. ‘There never was much hope,’ he answered. ‘Just a fool’s hope, as I have been told. And when I heard of Cirith Ungol——’ He broke off and strode to the window, as if his eyes could pierce the night in the East. ‘Cirith Ungol!’ he muttered. ‘Why that way, I wonder?’ He turned. ‘Just now, Pippin, my heart almost failed me, hearing that name. ...

Note Cirith Ungol means “spider’s cleft” in Sindarin

The hobbits only got through as a result of a huge amount of luck, several different things breaking in their favour, many of them as a result of distractions raised by Gandalf in distant places drawing the attention of the enemy away from them. So in this alternative future if Gandalf was with them, they couldn’t rely on any of that.

Surely not through the front gate? Going around by the South was too far. So did he know of another way through the mountains? Geologically it would be unusual to have such a long range of mountains with no other way through, especially for the likes of the hobbits on foot, able to scramble and with a rope. Especially since the mountains are also not that tall, Alpine maybe but not Himalayan. It’s possible - but they already have two passes through them, might they have more? Places not well guarded because only one or two people could get through at a time?

After all they had already shown their ability to get down a cliff with a rope, although with considerable difficulty. Gollum could do rock climbing easily. So they wouldn’t really need a well maintained path with stairs all the way up the mountains to a pass and down the other side. But Tolkien’s mythological geology doesn’t need to work exactly like Earth geology. Perhaps his mountains really are impassable over long distances like that?

Perhaps he didn’t have a detailed plan?

'We have not decided our course,' said Aragorn. 'Beyond Lothlorien I do not know what Gandalf intended to do. Indeed I do not think that even he had any clear purpose.'

And

‘Well, Frodo,’ said Aragorn at last. ‘I fear that the burden is laid upon you. You are the Bearer appointed by the Council. Your own way you alone can choose. In this matter I cannot advise you. I am not Gandalf, and though I have tried to bear his part, I do not know what design or hope he had for this hour, if indeed he had any. Most likely it seems that if he were here now the choice would still wait on you. Such is your fate.’

Or it might be, he had a plan that he didn’t share with Aragorn. Or he hoped to find out more on this way from Galadriel and his own observations that could lead to a plan.

Also Elrond seems to have foreseen that the Hobbits would be the ones to find a way:

“I think that this task is appointed for you, Frodo; and that if you do not find a way, no one will. This is the hour of the Shire-folk, when they arise from their quiet fields to shake the towers and counsels of the Great. Who of all the Wise could have foreseen it? Or, if they are wise, why should they expect to know it, until the hour has struck?”

He says “that if you do not find a way” not “that if you don’t make it”. It rather suggests that they could find their way in by themselves.

Would Gandalf have said “Okay this is as far as I can take you, my presence will alert the enemy and you have to continue by yourself?”. Or is that reading too much into that?

We have to remember that Tolkien is the master of the “flawed narrator”. So Aragorn saying that Gandalf had no plan doesn’t mean that in actuality he had no plan. Even Elrond wasn’t privy to what Gandalf actually planned, so we can’t really use Elrond’s advice either to deduce as a certainty that the hobbit’s would need to find their way. That’s just Elrond giving his advice based on many thousands of years of experience. He doesn’t claim to be able to foretell the future.

Anyway we know that Boromir would have gone to Gondor and so probably the Fellowship would have split up at around the point they did. Maybe Gandalf alone would have gone with the hobbits? Maybe with Aragorn. Maybe he would have left Aragorn and the hobbits to continue alone into Mordor?

Maybe his despair about them going through Cirith Ungol is because he wasn’t there. But he reacts as if they had some other choice not mentioned, saying “Why that way, I wonder?”. He couldn’t possibly be thinking of the Morannon as another way, the front gate of Mordor. So what other way?

You can also approach this as a story rather than as a fictional world. If Tolkien hadn’t come up with the idea of Gandalf getting lost in Moria and the party splitting and death of Boromir, what else might he have done? Tolkien talks about how he doesn’t know where the story is going. When it started in Hobbiton then in his early drafts Gandalf appears, with a cold, in place of the black riders - and his sniffle eventually gets rewritten as the black riders. He didn’t predict Balin’s tomb in Moria until the narrative got there, and then he stopped, stuck, not knowing how the story would continue, and in some grief himself for the loss of the fictional character. So when he wrote Elrond’s speech, perhaps he didn’t know himself how the hobbits would get into Mordor? So the uncertainty there, though put into the words of Elrond, may also be his own narrative uncertainty - that he felt that somehow the hobbits would find a way but he didn’t know how.

If so - maybe in that alternative narrative path, they’d have found another pass that wasn’t quite as impossible as Cirith Ungol, appropriate to a direct attack on an undistracted Sauron who was looking out for them?

I don’t think he’d have used the eagles. He’d done that already in the hobbit and it would have been a weak finish to the story. The objection of Sauron’s fell riders of the air, that they made Mordor a “no fly” region for the eagles while he was at the height of his power - that would not work if you think of him as a story teller because Tolkien could just write them out of the book, but I can’t see him doing it that way.

So - from the point of view of narrator, he could hint at other possibilities but there was no need for him to spell them out. So - perhaps he never did work anything else out? His book works by hinting at a vast history and enormous back story - and he had written some of that - unlike many authors he had actually written much of the non narrated backstory. But he certainly hadn’t written it all. His use of flawed narrators let him have a huge back story that none of his characters knew completely, and the fictional compilers of the “red book” which the LOR is supposedly based on in his fictional account of its origins also don’t know completely (had access to the libraries of Gondor but those can be flawed and incomplete too), and sometimes they’d get it wrong. And he has no omniscient perspective on his characters either - so others talk about Gandalf and see him in many different ways, like the young hobbits with their fireworks as the main things they see him as doing. But he never gives Gandalf long introspective thoughts about himself or his path. Sometimes Gandalf reveals his own thoughts and some of his past, but only in glimpses here and there. So the author also not only has a flawed narrators, but as in real life, you have flawed and differing understandings of the character, thoughts and background of those characters too, from multiple points of view. And to complicate that the whole thing is presented in the fictional universe as a flawed compilation of written accounts.

Those also are written accounts that we know can be incorrect in the fictional universe sometimes, as in Bilbo’s first written account of his finding of the ring. In the original version of the Hobbit, Gollum and Bilbo do have a genuine guessing game and Gollum is dismayed when he can’t find the ring to give to Bilbo, and they part amicably. Later Tolkien had to rewrite it to fit the LOR, and so we get the new story, and the old story is presented in LOR as a lie that Bilbo told to strengthen his claim on the ring, under the influence of the ring. So then those two versions of the Hobbit become alternative flawed accounts of what happened with the first one definitely wrong. So what else might be incorrect in the two accounts? You can’t really say that everything in the LOR or the Hobbit actually happened as described in the fictional universe, though it’s a reasonable first guess. Also the characters themselves speculate about how their own adventures might be written up in the future as Frodo and Sam joke about it.

So Tolkien is having fun with flawed perspectives here at multiple levels.

So, I don’t think we will ever know. And quite probably Tolkien himself couldn’t have said, if asked. I think only way we could have new light shed on this is if someone turns up a letter or a written out story that sheds some light on it - but if there was any such, surely the Tolkien fans would have turned it up by now??