Ok that's information I didn't know with the putty, so appreciate that. I did just find the specific brand just after you answered so I'll check their information sheet on whether to putty/silicone or not and order the shoe gasket through them. Thanks to you both! parts.maax.com/zMAAX/public/PartsOnline?country=us&lang=en#

I did, yes. After cutting a hole from the downstairs ceiling, I was able to identify it coming out directly at this connection from below. The previously installers luckily had a section of the entire subfloor cut out (maybe for similar reasons?) which made it easy to see straight to the piping from down there. The first image shows it slightly, but the metal plate that the pipe goes through is completely rusted out, so that is another sign it's coming from above it. I've added an additional 4th image just now showing it from the bottom. Came from a small hole in that rusted plate.

Ok thanks. I couldn't find any markings of a manufacturer on the spout or handles unfortunately, so I may go to a hardware store with the spout itself to see if they'll be able to assist. These spouts are crazy expensive for some reason, so I'd rather go that route if I can find a proper fitment to feel comfortable again. Anyway, appreciate the affirmation with additional prevention and confirming the "o" ring is actually the only required measure. Seems odd for as the only failsafe to drip into a completely inaccessible space, but I'm no expert obviously :) Thanks!

It was probed for how much solid was in the tank, and the company who did the regular maintenance was contacted for recent pump which was 1.5 years ago. Inspector, who was hired directly through my lender and so I cannot verify his qualifications, said findings were adequate.

Appreciate it folks. I'll mark this as the answer. If anyone knows how I'd figure out where the slab may have the shower pipe, I don't see any markings hah. Anyway, very helpful and thanks for confirmation!