Citation needed. It seems pretty clear that a mechanism to allow a user to access a battery will increase complexity, making all the other properties harder to achieve.
Fairphone managed to do it, I'm sure companies with more budget than them can figure it out.
It'll increase the size of the case by a small amount but a battery cell is a battery cell... Rip open an old device and you'll see.
You're asking for proof that effective waterproof phones with removable batteries exist?
https://m.gsmarena.com/results.php3?chkRemovableBattery=sele...