Oops turns out that was actually it. Totally missed the whole thing with the Nun. Thanks for the help! Finally beat the game and now I'm on a second playthrough and running into the same issue with the water girl as the guys earlier in this thread.
Edit: Managed to finish the NTR route using an older save I made. Now I'm just missing the 2 CGs after the good ending in the gallery (I think they're the threesome CGs with the generals?) and have no clue how to get them.
I believe the option to open the gate only activates when you are at that part of the game. Trying to open the gate beforehand doesnt allow you to, regardless of whether you have key or not. For the H scene with hobo, IIRC you need 150-250 Corrupt bar. Choose first option.
And as far as I can tell, the final game option depends on how Sphillia feels about MC? I completed all sex scenes between the two of them, and the first option unlocked, but the second option unlocked otherwise? Can anyone confirm this one for me, just to be sure?