I've been sitting here for an hour trying EVERYTHING I can think of, but I can't get the tentacle monster (third stage) away from Saiko. The dude just says "This has gone on long enough" and that I should get it away from her, but there is literally nothing I can do. I can't move the monster, I can't move Saiko and there are very few props in the lab to work with... I tried moving the monster with the chair, and that didn't work. I tried putting the chair between the monster and Saiko... Didn't work. I tried turning on the projector and nothing happens. What the hell am I supposed to do? Nothing happens and there are no hints. Am I missing something?

Hey there, I got stuck here too, (even skipped the section first time lol) what seems to work is walking in between and trying to push it away from her (may take quite a few attempts) hope this works

Stand between the creature and Saiko and he should go away.

I did that about a hundred times. Eventually it randomly walked away from her when I happened to bump into it from behind. (Note that I had already done this many times before this point and nothing happened)

Now I'm stuck at another part. The dude got tired after the monster fucked both the girls at the same time, I went to bed and now he's been sleeping for over 5 minutes... Nothing is happening... I can even go to the city overview and he still keeps saying "zzzz... zzzzzz".

You have to forward time in the city for about 8 hours to wakeup again.

I ran in o the mosters front from bed side then it moved away.

After getting the creature away, Saiko is pissed at me, in basement part

Well, yeah. You did chase away her sex toy after all.

Even in the dungeon after story

If you're talking about the fast sex dungeon, that should definitely not happen. That shouldn't even be possible.