ok trying to keep this organized
1) No idea how they work it in terms of price, but I do know that tirerack offers prices less then typical wheel retailers and they end up using them with special contracts I beleive. I don't know much more about the technical behind the scenes details.. and again, I have no idea if your retailer does that at all.
2) 18" wheels in gernaly fit on the E90 just fine, as we all know the BMW comes standard with 17 and 18" wheels. Brakes are not too large, etc etc.. I suspect it has to do with high offset on the rear, which is more of a visual thing then anything, you can put spacers to fix that, but most wheel sellers wont recommend that, nor do I when you have the option of getting the larger rear wheel.
3) 255 tires can fit on a 9.5" wheel, its probably most ideal on a 9" wheel but it can fit on a 9.5" wheel, i believe 255 bridgestone tires are rated up to a 10" wheel. You can use a 8" or 8.5" on the front, that is your choice really.. Not much difference more of a decision based on tire size.. many trackers decide to go with the 8.5 on the front because they run larger sized tires up front (245s), something u may not be doing yourself
4) Yea Peter Lee from wheel experts is a good guy, he should be able to get you the right wheels. Maybee send him a followup email if he doesnt get back to you.
5) I dont know anyone specificaly in SD, but I will PM you with some names in your area (cali)