I'm not sure just how much more backspacing you can go with the 15's, I noticed that my calipers are already close enough to the rim, you may get another 1/4 of an inch but, would be very careful, about altering rims for offroad use, I have used altered rims on road, on my old mini, but that was a long time ago and as I said, onroad.
Best bet is to put one of the 15's back on, measure the clearance from the caliper and then show that to the place that can do the alteration. Another idae is to see if someone can do a new wheel alignment with a castor adjustment that pulls the wheels forwards, that can give you extra clearance from the pinch weld.
Big bumpstops in the back stop the rubbing and guard/tyre damage there. To see some pics, check out my thread in the "KJ 4x4 Discussion" area about my install of new replacement rear coils, there are some pics there of the rear bumpstops.
http://www.lostjeeps.com/forum/phpBB3/vie ... hp?t=30211
Seeing you are also thinking of getting more lift in the front, that in itself coulds solve the rubbing issues you have, and seeing you still have the tyres lying around, would make sense to see if you can get them to work, as you will probably have to do the exact same things to make any other large size tyre combo oto work as well.
You did definately get the 7inch rims and not the 8 inch rims Ash???? Just asking , because if they did supply you the wrong ones that would explain everything.