My advise is to throw out that sealant kit, at most only keep compressor.
1) most tire repair shops refuse to deal/repair tires with that gunk used on,
2) that kit won't help with all puncture types, so i wouldn't call it something that i can 100% rely on,
3) very probably you'll have to buy at least two new tires for tires of same wear to be on axle (at least that's what i'd try to at least in rear, due LSD diff), but if punctured tire without that crap used on is brought to tire shop, it often can be repaired for relatively cheap and it will be of same wear as rest of tires on car.
Imho full sized spare also has it's cons, eg. it's extra weight, and also robs space from already small boot, due sticking out
So .. i'd just spend more time trying to find spacesaver that does fit over BBK. For example here and here, spacesaver that fits over performancepackage brembos, P/N 28151FE030 (+foam spacer 97046CA000 (under wheel), +foam insert 95086CA070 (different from smaller non-PP insert), and just tire P/N 28121VA050 T145/70D17).
Here i found for example 18" spacesaver of 5x100 pcd. Maybe it also can provide better brake clearance. If nothing else, then with spacer.