About exp. points...
If you have yet to unlock the Mastery system, your XP goes nowhere. Same place it goes when tracks are full and you have not enough Mastery Points to complete another, or haven’t unlocked another track.
If you have fulfilled every Mastery track in a region (Core or Heart of Maguuma), your XP will again count, and award a Spirit Shard (or two/three?) at the end of each level-up.
Good luck.
you can work on different mastery tracks, one at a time
when I was short on maguuma mastery points, I worked on each until it was as far as I could earn, and by the time I could afford to promote, I already had the necessary XP.
the other thing you can do is hold onto any “daily” chests until you have the mastery points; the ones from bloodstone, bitterfrost, and ember bay each award maguuma xp as you open them