My 6906 slide goes on easily, but the slide on my 3913TSW absolutely will not go back on unless I pull back on the hammer slightly to get the third lever (believe its the firing pin safety lever - the one closest to the hammer) to go fully into the frame slot.

Can't help with the mag release, but there is a good youtube video on complete disassembly of a 4506 which will show removing and reinstalling the mag lever. I think it can be adjusted with more or fewer turns on the mag lever screw. Here is the link ttp://www.youtube.com/watch?v=IEY0C5KTh_8&feature=mfu_in_order&list=UL. If the link does not work, just do a search on youtube for 4506 advanced take down.