My experience with spacers has been limited to my last two e46 M3's. And, since I can't answer all of your questions, I won't attempt to try. However, my experience has shown me that installing spacers can have both positive AND adverse effects regarding both under and oversteer.

I know this sounds vague. But in the case of the e46 M3 it suffers from a lot of understeer. I installed 10mm spacers in front only and noticed that it reduced the understeer a significant amount. I then dropped the rear 1" and just about eliminated all of it to where she handled very neutrally. However, if you install spacers both front and rear of equal amount, you negate any effect one way or the other.

Relative to bolts, DO NOT use stock bolts. They will wind up being too short and you can possibly damage your X and yourself if at speed. Pay the difference and get the proper length bolt which will be determined by the thickness of the spacers.

As far as vendors, there are several. I think I bought mine from Zegenhaus (sp?). I think Turner sells them also.