I think it's great that this has been a fan conversation that's been going on since literally as long as I've been able to use the Internet as a pre-teen. Surely all that could possibly have been said and speculated has been said ... and yet we've all come back to add our two cents one more time
The portal to Hell is clearly THE intended secret in Gilbert's "Mutiny on Monkey Island" pitch document. That document is really just three pages, and mostly sets up a). the treasure of Monkey Island as a mcguffin and b). the twist that there is no treasure, it's really a portal to hell that turns pirates into an undead army. That seems to be the pitch that Gilbert started with, and everything else built from there.
However, I think during the design process, this original 'secret' began to feel less important as the characters, jokes and puzzles became compelling in their own right. The structure of "treasure hunt as a mcguffin that leads to a third act twist" ended up being a narrative hook to hang all the fun stuff on, rather than being the main conceit of the game. Heck, in the finished game, the 'secret' is dramatically undercut by the reveal of LeChuck's ghost ship on a river of lava under Monkey Island at the end of the first act. When you finally get to Monkey Island, Guybrush is almost nonchalant about the 'hot breezes of hell' ... I think this twist was no longer a big deal to the developers, and they may have even forgotten that it was supposed to be THE secret of Monkey Island.
However, as a kid, the stuff about Hell in the first game spooked me SO much. I remember laying awake at night after that first cut scene with LeChuck and Bob and feeling that I had witnessed something unholy and deeply wrong. I only returned to it because my parents were ahead of me in the game and said it wasn't too scary. And there's enough silly stuff undercutting the horror of the original 'secret' that it was palpable to keep playing.
The second game I mostly played on my own, and I did NOT understand a lot of the subtext. Since the secret of the first game had been so horrifying to me, I remember being almost giddy with dread at the prospect of experiencing whatever Big Whoop was. I vividly remember pixel-hunting over the smashed treasure chest to find ... an "e-ticket"? I was too young to have understood this cultural reference, and it totally befuddled me. I had to get grown-ups to explain what it was, and still felt totally confused. I understood the self-depreciating meta humor of "never pay more than twenty bucks for a computer game", but this was all too oblique for me as a twelve-year old.
In hindsight, I think the original intention in MI2 was to reuse that same narrative structure of "treasure hunt mcguffin leads to third-act twist", but with the understanding that both the treasure AND the twist could both be kind of hazy since they weren't really the main point of the games. The things that vexed me most as a kid are, in hindsight really just obviously lampshading that the plot is arbitrary. The deliberate pointlessness of Big Whoop is almost obnoxiously obvious, but because the first game had such a strong central twist I refused to believe that it was meaningless. My hunch is that the 'e-ticket' was an in-joke reference to how the design team regarded the ending sequence before it was written: the big finale that we'll dream up later. Gilbert has said that he didn't know how it would end until basically the last minute.
I think the truth is that Hell was the secret of MI1, and the secret of MI2 was that they started making it before they had an intended ending. Ron managed to come up with a twist that simultaneously built on and undercut the dread I felt from having experienced the 'true', hellish secret of MI1. It also managed to make the jokey unreality of the first game feel retroactively creepy, like the Grog machine and the circus were also somehow portals to a nightmare. None of the other games has recaptured that feeling for me (although I think the Rise of the Pirate God chapter of Tales came closest).
There is no 'original intent' for the ending of MI2, because they started the game without a clear ending, or intended twist. I suspect the theme park stuff started to seep into the first game during the design process, but it had just been a background element that got pulled to the foreground when a twist was needed. I think Ron Gilbert is happy with the improvised, ambiguous ending that the game shipped with, and after all these years, I am happy with it too.