Correct with regard to your first scenario where 2 partitions never talk to each other in the future, you dont need to consider it. If they do talk to each other in the future, and have to merge, this is where Bitcoin, blocks, POW and longest chain rule falls on its arse. Only one partition can exist, there is no merge possibility so the other has to be destroyed. Even if the 2 partitions have not existed for an extended period of time you are screwed as they can never merge without a significant and possibly destructive impact to ALL historic transactions prior to the partition event, so you end up with an unresolvable fork.
I think you mean 'post' partition event? But yes, I agree this is a bit brutal. However, on the limit where each partition spends the same outputs as the other, this would be the net result anyway. I guess any new design ought to be able to merge partitions which were completely unrelated, which is what, I agree, longest chain does not.