#1. Yes, you can change them out for another sort of spring. But I would say first look at the attempt to flip the axle over and allow the spring to come from under the axle, then use shackles on one end of the springs to allow them to move. I've done this many times it works great, you should still have a couple of inches between the frame and the axle for spring flexing.
#2. Yes, I would build the pit first, then place it on the trailer and adjust according to the real weight so it is in the right spot.
jm2cw