just get a real set of wrenches
multitools are for roadside emergency use at best and are too short to get propor torque for bike assembly
using a torx bit where it shouldnt be runs the risk of stripping the bolt...you know how tight the seat was on initially? thats how tight you should put it before riding; get the right hex key for doing this ....

suspect seatpost is going to be the least of adjustments needed -there's a reason bikes at the LBS cost more than from a box; and it's worth it