"Don't know where you got that it was under a specific old volcano, but Hephaestus was built around a series of geothermal vents and volcanoes that its harmonic cores harness" ---
I said on an ancient undersea volcano - one for the geothemal stuff and two to raise the base depth something alot closer than 2000 foot depth of most of the seafloor in that area.
"and the trees as oxygen as "possible, but not too smart"." --- if you could accelerate their metabolism as was mentioned in BS1 'frankentrees" using ADAM base genetic changes they would make efficient Oxygen converters (and produce useful byproducts as well). Mr Science talked of CO2 Scrubbing but those work using a consumable compound that has to be continually produced and renewed (which would largely stop when Rapture fell apart).
"most of Rapture's buildings are built somewhat far from that area." but if you look out the windows AT Hesphaestus you see buildings (besides the Hesphaestus complex) in the immediate vicinity.
"but that doesn't mean there are arched windows above the ceilings and in the tunnels, and some areas do have some rounded off windows to corner off the walls" . Which would work better physically/structurally but the problem windows ARE still the big flat ones. In engineering it is what WONT work, not what does that kills the project.
"The host did mention that the stone statues and sharp-flat Art Deco architecture wouldn't last long from pressure" The pressure is irrelevant as metal and stone dont 'crush' (they dont have air chambers inside). They are more likely to be effected by slow corrosion and would have been made of something better than painted steel like the Titanic. Same for ocean currents which OI would assume were already compensated for.
"but if they're destroyed by war or pressure damage, like what happened in BioShock 2, relying on them as" - its all dependant on the power being kept running for air (has to circulate even when renewed) and for temperature (else the place would have the temperature of the seawater outside), and secondarily for lights and any machinery that works (fun game if you had to pry every door open). As for general damage, with only a remaining tenth of the planned population (and the original installation having a typical safety margin of 300%) you could have most of the systems wrecked and still support the people we saw in the games (and likely with someone under Ryan or Lamb still at work eeping some of the system running).
"No theory is always going to be 100% perfect" -- when you start building things you better have a better grasp than just theory and the engineering had BETTER be close to 100% as things fall apart very fast and underwater or out in the ocean where one wrong step and you drown . Anything less is just a disastrous deathtrap that fools might build and bigger fools risk their lives to live in.