Ignoring the fact that you'll probably end up with a genetic mess, that scenario is so fucked up regardless of who the last two people on earth are the best thing to do would be not get pregnant and just enjoy whatever time you have left.

Not necessarily, it obviously depends on the genetic stock of the two left, and any genetic defects will be multiplied by this situation, but it isn't a guaranteed death sentence. Whilst no where this restricted there are species in the world that have recovered from near extinction. A small population, without general healthcare or large society will have an accelerated evolution too, with shorter generations and a higher risk of death due to the genetic issues from your original stock.

If we assume that said human will be prey for another animal, too, we would see only the healthier humans making it past childhood.

But either way it would be grim.