The B Pillar trims will come off and go back on without removing the seats but it's easier to get them back on with the seats out. Yes the seat belt mountings do have to come off, if you are taking the top one off treat the plastic cover with care as the hinge part of it breaks off very easily! If you remove the belt tensioner from the body remember that they are a pyrotechnic device and can tension if ill treated, I've yet to hear first hand of it happening but there is always a first time!
If the fixing pegs remain on the body note that they are not all the same so position them back correctly.