One thing that can be done in case CD Projekt is unable to give us the missing XP through code on the next patch: give us a neat table of how much every specific quest rewards you with (assuming there isn't one already). That way you can add up all of your completed quests to determine how much XP you should have based on quests alone. I, for example, have a manual save that is more or less where the problem started happening. I can compare it to my current save to determine which quests I've completed in that time. When the patch fixes the issue, I calculate the XP I should have received from said quests and use a cheat to give myself the progress I'm owed.
That information could be given to us in-game: in the quest log, every completed quest (and only the completed quests) could contain the total XP it rewards you with, plus the level you were at when you completed it (so you'll know whether or not you should receive XP for it). With that information, we can calculate exactly how much XP we should have and give it to ourselves. A bit messy, yes, but a better solution than replaying huge chunks of gameplay.
EDIT: It just occurred to me that this would only work on the PC version, so I suggest something similar to this be offered to console players if the missing XP can't be retroactively given. Maybe a one-time option to give yourself a certain amount of XP. Again, messy, but better than ruining their playthrough.