Then ABC chain will be fucked up... with or without checkpoints.
naah, upon restoring of outside connection the miners would just switch to working on the longest chain irrelevant of the period of separation (when checkpointing is not in play)
Yes, with no checkpoints miners will switch to the longest chain. With checkpoints, that will lead to a chain split and human intervention will be needed to solve it.
Yeah, but in that catastrophic situation, a chain split it's even possible without checkpoints: miners just have to manually declare invalid the Chinese chain.