Here's one suggestion, actually. Given, it would probably require an overhaul to the manner in which XP and SP are given during the process of a leve, but it would both take care of the disconnect issue while hopefully not allowing for abuse of the system via disconnecting, then reconnecting over and over.

Make it so that any and all XP/SP collected through the leve process is not awarded to the character until the leve is completed. Now I know some might call foul on that, but keep in mind a few things.

Unlike other MMOs out there, gaining a level does not automatically restore your HP and your MP right away in FFXIV, so even gaining one mid-leve doesn't make much of a difference to the player anyway. In my own experience, and maybe others do this as well, I don't often even touch my new points to disperse for a physical level until after a leve anyway, when I don't need to worry about a timer, or seeking out monsters, or what have you. As for any missed, new job abilities, well, if you are able to finish the leve with the abilities you began it with, no need to be impatient to get access to that new ability from your new class rank for just a few more minutes, right?

The XP/SP reward would still be the same amount you gathered during your leve run, that is to say you would still get XP/SP depending on what you kill, and not a set total at the end like how other MMO quests do with XP-related quests. And you would still be able to see just how much you are getting whenever you kill a monster or do something that gives you XP/SP during gathering, so you're not left wondering how much you've earned until the end of the leve. Hell, it would even give you a tabulated total of the XP and SP you grinded at the end of the leve, for added convenience.

Just an idea, though.