I just realized that Rystil had been awarding XP along the way, so it's only the last, unfinished encounter. That makes things easier. Jace and Quozen each receive
250 XP for the dispatch of some of the zombies
100 XP roleplaying bonus (reflecting the whole game)
1000 XP for time spent (2nd level, 10 months)
Total: 1350 XP
Now, how to handwave the storyline? How about this:
Quozen and Gloria manage to bring reinforcements from the monastery, who are decisive in repelling the undead, killing many of the zombies, but not before Valan, Dorrin, Hasin and Droggle fall, and Jace barely survives. [Note: If the players of one or more of those four do decide to return, I'll amend that to allow their characters to survive too.] Taken in to safety, Jace and Quozen explain their mission. After some discussion, the abbot decides that it is too dangerous for you to carry the shard back. After all, the undead were chasing you right up to the monastery. Since the enemy is a death cult, it is possible that this ghost was under its control, and it remains out there. The monks will send a team to bring the shard to Monemvassia. But not yet: After all, if the zombie army was in fact under the cult's control, they could be watching. The monks will sneak out later, when they might be unexpected.
Quozen and Jace may decide how long they'll spend in the monastery (the monks are quite hospitable). Whenever they do leave, their return trip turns out to be less eventful than they journey out.
Is this narrative reasonable?
Thanks a lot for your patience sticking with the game. You're free to bring your characters back into play now if you choose.