I too have an alpha pioneer dc plus. The towers sliding is a known problem, and sadly, quite common. Regular posters will chime in on their unique solution, but I just put a piece of old leather between the towers and the base, tighten the knobs, and problem solved.
As for mounting, I just make them snug in the beginning. When I pull mains, the racquet gets shorter and I notice a little slack in the top and bottom mounts, but that's no problem. When I pull crosses, the racquet gets back into shape. I once tried tightening the mounts to take up the slack but when I pulled the cross, the racquet stretched so hard against the mounts that it developed a hairline crack (maybe just the paint). So I don't mess with the mounts anymore while stringing.