Ranger first is a solid choice for marching order. Ideally you want a Rogue, but a Ranger works too.
Yeah, that's the general reason why traps aren't that good in your own base. They prevent your own people from moving around.
Probably a DC 10 check. Could be 15. And of course the octopus notices, because he's awesome. Or she, I forgot.
Most attacks don't have saving throws. They just contest the target's AC.
Funny, but kinda dumb use of the Enlarge spell. They're smart enough to use weapons, so having your vision go up to more or less to the top of the door should give you at least a pause.
Lots of enemies is always dangerous, unless they're significantly weaker. Action economy is important, and numbers can beat a high AC difference.
Mimic induced butt shot. What is this, Frieren? Well, I won't complain.
A lot of DMs would have that destroy some of the loot. Actions and consequences and all that.
when the octopus have more perception than any of these guys
Well, for the roll, yes, but in general in the middle of the pack. In general a manga won't have the same kind of swinging results that would occur in real life games, though. Rolling a convenient or inconvenient 20 or 1 can easily come off as an asspull.