How can you remove a dent in the tube? Check if stem can be adjusted. The seatpost is painted, so that should make things a little better, just install the old seat, loosen and remove the seat post bolt, wedge a fat screwdirver or what-ever fits between the "ears" of seat post clamp, and try spreading it a little bit, add wd-40, and repeat.
Than stand over the bike facing backward, and try rotating the seat with all the strength you have. I hope that helps.