If you get a bunch of Sigs, you'll have to get used to them, and that may affect your performance with other styles, that's just something you'll either have to accept, or train to get around.
I don't know about the other guys, but I know for myself I'll probably stick mostly with Sigs. I don't think the Sigs are the be all end all, and I do shoot other make/models whenever given the chance, but I always find myself going back to my Sig and enjoying it the most. So I'll be getting more Sigs are they're the best for me.
First off, once you decock a Sig, it's not partially cocked. The small gap is just the normal resting state.
1) All the DA/SA, DAO, DAK Sigs have automatic firing pin blocks (not sure what the SAO have), that can only be disengaged by the trigger. Try this, when the Sig is decocked try to push the hammer till it hits the firing pin. It won't go far enough. With the PISTOL EMPTY, cock it, pull the trigger to decock it and hold the trigger down. Notice how the hammer goes all the way forward to the firing pin. Let go of the trigger, let go of the hammer, and you'll hear a click as it reengages the safety.
2) I don't see how change could possible fall in there, although I suppose pocket lint or toothpicks could. Though I wonder if either of those could stop it from firing. I guess my solution would be, don't carry it in your pocket? It's a little large for that it seems to me. From what I've seen the DAK models have the hammer recessed into the slide a bit more which could help.