If your character is transformed into a monster by true polymorph, can you legally continue playing them if the effect isn't dispelled?
If not, then what are the ways to be reverted if the session already ended?
During the session, you can keep playing the character. It's still the same entity/person, because True Polymorph says:
It retains its alignment and personality.
After the session, you can't stay polymorphed because the Season 7 FAQ updated the rules on True Polymorph:
The following guidance applies:
Dragons Aren’t Allowed in Town?! The effects of true polymorph are dispelled at the end of the adventure or session — whichever happens first.
1It's a "no" under formal logic - the statement of "legal character AND stay True Polymorphed" must be true in both parts for the answer to be "Yes". You can't fulfill both parameters (the second is impossible), so the answer to the question is "No".
No, once you've been hit with true polymorph and it's become permanent, you need to get rid of it to keep using that character. From the Adventurers' League FAQ:
Dragons Aren’t Allowed in Town?!
A character that ends an adventure as a race or creature not otherwise available through their character creation rules is removed from play until the effects of the true polymorph spell are dispelled.
You can. As of Season 7 FAQ, True Polymorph is automatically dispelled after every session/module, so you will be reverted to your normal self. If you want to become yourself prior to the end of the session/module, someone will need to dispell the effect.
If you find yourself too big to fit somewhere, you will likely have to wait on a mountaintop somewhere while the rest of the party does its thing. If you can't join them because they went somewhere you can't you'll just have to wait until the session ends and continue along the next time as your normal self.