What I don't get is why did the rednecks keep Castle and Beckett alive, did I miss that somehow? They certainly had no issue killing the truck driver, so I don't know why they wouldn't do it again.
Possibility 1: They didn't want to bring down the trouble they'd get from killing a cop if they could avoid it.
Possibility 2: They're fans of Castle's books and wanted him to survive to keep writing them.
...and they moved that hatch really easily after it was made clear they weren't able to even push it up against the wall in the first place. Unless they removed all the chains that were inside before propping it up onto it's side.
From context I take it you mean "they moved that trunk really easily." If you've ever had to move a heavy box or other object across the floor, you may have found it easier to tip or roll it onto its side than to slide it flat across the floor, because you aren't fighting as much friction and have the advantage of leverage. (In fact, friction works in your favor there because you want the fulcrum to stay where it is.) Then once it was up on its side, they could've "walked" it into the desired position under the hatch.