Is a 1" carbon steerer + 1/8" shim + 1 1/8" stem acceptable or likely to end in disaster?
100% fine. You're supposed to position the slit in the shim towards the front of the bike, i.e. on the opposite side of the steerer to the stem bolts, but apart from that: nothing to worry about.
100% fine. You're supposed to position the slit in the shim towards the front of the bike, i.e. on the opposite side of the steerer to the stem bolts, but apart from that: nothing to worry about.