BigDaddy,
Same story here: My 6904, 6906 and 3906 are similar to yours with respect to reinstalling the slide. My 3953 goes together much easier - as it should. WRT to the magazine release, the condition you report is not normal and you should investigate further. Inspection and cleaning, followed by proper lube would be a good place to start. The mag release is also adjustable, and I believe can be reversed for LH shooters.

Interestingly, I just purchased a SIG P225, that was imported after serving with the Austrian police. It was REALLY dirty: I pulled the grips and mag, hosed it down with breakfree, wrapped it in a paper towel, and put it in a freezer bag for a couple days before I even began to look at disassembly - It' amazing how dirty some folks will let something get. Particularly curious since they're betting their life on it.....