Sorry, I wish I knew more about this stuff. But it sounds like your reasoning is, well, sound. The "endorsements" from PBs regarding neighboring pools is somewhat encouraging, but not really any sort of proof of anything. How old are the pools? Have you seen any of them? Can you? (If it were me, I'd let you in to look if you were my neighbor. Go ask.) Any evidence of them "sliding" down the hill? Man, I'd really want to look at those other pools and decks.
I can certainly understand not wanting (or being able) to pay for a massive foundation solution. But you can't not find out if you need that solution or not. That's sort'a like not going to the doctor because you don't want to hear you're sick. I think just your hunch about this is reason enough to have a proper engineering report done. It's odd that the other company backed out. Oh well. Find another (or the original one, as suggested). Basically, you don't have enough data at this point, and personally I wouldn't feel comfortable with an "engineering sign off" from a PB that basically reads "Oh, it'll be fine. We do it all the time!" Maybe it will be, maybe it won't. $30K is one thing, a broken pool is another. And a PB's lifetime warranty on the shell (if that's what you'll be getting) sounds comforting, but that not your lifetime, nor the pool's, nor the lifetime of the PB. It's the lifetime of his company, and more accurately... his bank account.
You need an expert.