theres a tool called an impact driver that is a screw driver you hit with a hammer to turn it... sounds strange if you've never heard of them, but work well on old screws without wrecking them. you have probably already sprayed wd40 and such on them?

shouldn't have to remove top of back seat to get bottom out, but there is a bolt that should be near the centre on the passenger side that retains a metal strap, this holds the back of the bottom of the seat in, you should be able to reach between top and bottom and undo it (if its still there) the only other thing holding the bottom piece is two tabs that slide down from the front of it into the floor, they just need the front of the seat to be lifted straight up