Can I use a GFCI on an ungrounded 2-wire circuit at the terminus?
The wire needs to be designed to handle the capacity of the circuit it's attached to, not the device that is plugged in. Otherwise, if anything is plugged into the end, or if there is a short, that draws more power, but not in excess of the circuit breaker, you would have a fire risk. I don't have a copy of the NEC, but to the best of my knowledge, in the US, a 15 amp circuit needs a 14 gauge wire, a 20 amp circuit requires a 12 gauge, and a 30 amp circuit requires a 10 gauge. Therefore, your 18 gauge wire is insufficient to attach to any standard circuit breakers that I've seen.

To answer your second question, you could install a separate ground for the other end of the connection and wire a gfci to that, assuming that the existing wiring were adequate for the circuit.

To fix the conduit, I would eliminate any sharp 90 degree turns and if it runs over a long enough distance, break the run by going up to an access panel.