I agree with Valiant Lucy.

Though I doubt Eustace would be able to talk to Caspian, as both were unfamiliar with Narnia. It is possible, though.
Puddleglum probably wouldn't have gone with them if there were many adventerurs.
If many Narnian adventurers had accompanied the 2 and not Puddleglum, Eustace's and Jill's faith wouldn't have been tested as much, therefore not teaching them as much of a lesson. So I believe Aslan intended for it all to fit together like this.
It is possible that if Eustace remembered the signs, they wouldn't have been convinced by LotGK to go to Harfang. But, from Harfang, that was the only place where they were able to see the lettering: "Under Me." I don't know how and where they would've noticed it otherwise.
And if the adventerurs were somehow at Harfang, the group most likely wouldn't be able to fully escape without waking giants.

I think it would have been much easier for Jill and Eustace if they had gotten to Caspian in time. They still would have eventually found their way to the ruined city, but perhaps would not have ventured to Harfang. (They probably wouldn't have needed to go to Harfang either. As King, Caspian could have ridden on horseback with an army through the Giant country with no or few setbacks. They would have gotten to the ruined city much earlier in the year, and they would have brought tents with them to shelter themselves.) Soon they would have found their way to the witch's realm and doubtlessly would have found a sneaky way in to get Rillian.

Hmm... Are we discussing what would happen if everything that didn't happen did? There's a lot of stuff I recall. It would have been a lot different for sure!

Well, if they had managed to talk to Caspian, he probably "would have given them an army to go in search of the Prince." I assume they "would have started at the same fountain where the Lord Drinian saw the Lady." (Puddleglum probably wouldn't be thought of to consult, and I'm not sure the owls' meeting would have taken place because I thought it was arranged by Glimfeather to decide what to do after King Caspian set sail again, and Caspian would probably have remained at Cair Paravel if he managed to talk to Eustace.)

Anyways, if Jill, Eustace, and the army travel on the same route they do in the real book, they probably would not have met the two travelers on horseback

because if I were the Witch, I wouldn't try to meet a huge group of Narnians. (Although, if she did, and if she knew they were coming after Rilian, she probably would enchant them or do something. )

After that (assuming they wouldn't meet her), if they stick to the Signs, they probably would find "a trapdoor" or something. Aslan would have made a way, that is for certain! Then they would sneak into the Underworld, and there would probably be a fairly large-scale battle that occurs between them and the gnomes.

And after that? I'm not sure!

It's all well and good to hypothesize how the book would have ended up had Eustace and Jill obeyed Aslan in the first place, but, in my opinion, that would ruin the central theme of the book (Not to mention making it incredibly boring and unrelatable to just about every reader).

When I read SC, the main thing that I see are themes of disobedience and forgiveness. Despite the fact that the two children disobey Aslan throughout most of the book things still turn out all right in the end, which just goes to show how forgiving Aslan is, and how wonderful obedience is in opposition to disobedience.

If the whole book was about nothing but "Do as your told and everything will be okay" I suppose that one could get a message out of it, but. . . would it really apply to us?

Who among us hasn't disobeyed God's commands? Who among us has never disobeyed his or her parents? Who has never reaped the consequences of disobedience. You see, that disobedience is the central theme of the book. It's the quality of that book that makes it so relatable to all of its readers, and it's also the conflict which the plotline later resolves, thus bringing closure to the story.

I have always thought that if Eustace had heard the signs from Aslan, he would have talked to King Caspian on time! I wish I could change the story.....but then where's the fun in that! then none of the crazy stuff that happens in the stroy would have happened!

Yeah I think everything would have changed, no Puddleglum
Jill probably would of waited at the castle with the dying king as Eustace would of taken Caspian's army up to Ettinsmoor. My guess depends on the Harfang and the Autumn feast. But yes the course would have changed.