The mountain and boat seen in Typhoon Lagoon likely contains a lot more steel than the one in Splash, since it was designed from the beginning to support that boat. There also aren't any slides or guest areas directly under it and inside of the mountain like there are in Splash. Just mechanical and pump equipment. I think only cast and maintenance go near or inside/under the boat.
That said, it sounds like this was more of a money thing more than an impossibility. And perhaps a little re-engineering work could have even alleviated a significant amount of those money problems without compromising as much on the visual impact.
The tree for instance doesn't look like it would have been that big of a problem. Just a few extra branches and some foliage that I doubt would add all that much steel and weight. The boat may have been more of a problem, but one idea I had was to remove it from atop the tree branches and relocate it further down to serve as the entrance to the cavern. This would even arguably make a bit more sense within the context of the ride's scenes since the peak of the lift interior is supposed to be the inside of Mama Odie's house.
I'm not an artist, but I made a very rough edited mockup for what i'm referring to.
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