They've fought regenerating enemies before so while they don't know the squid's weak spot I imagine they will try to shred it into as many pieces as possible. Which probably means all tentacles separated and the main body sliced apart as well, oops. But if you think they would've missed some, feel free to roll for it?

They would indeed like to check out the treehouse! They'll be sus at first (especially what with those man-eating houses they've heard about) and check it from the outside first, then through the open door, probably throw something a peach in to see if anything happens, but if all of those steps turn out clear, one of them (probably Hange) is going to enter it to get a better look inside while one of them (probably Levi) hangs outside to keep an eye on their surroundings and also make sure that door doesn't suddenly slam shut.