OOPS, I must not have refreshed my screen in a while. Glad you got it apart without driving out the pin. I can assure you though, if you were able to get that slide stop spring back in place by yourself, the pin would have been a piece of cake.
Just a guess, but I'd wager that the slide stop spring wasn't all the way into the groove. If the trigger guard latch overlaps that little spring, it is on there TIGHT. Like I said, that's just a guess, but it would explain why the spring came out.