How much of a stat bonus can the characters get via revocation

No. You can receive and use treasure maps as soon as you are able to Canvass for Guests at the Quester's Rest in Stornway.

If you receive a treasure map (whether through canvassing or the normal quest in Zere Rocks), you can share it through canvassing as soon as you have beaten that treasure map's boss.

No, there is no downside. In fact, its a very handy way of getting some early skill points by switching jobs and leveling up very fast by hunting metal slimes, metal medleys, lmses, etc. (When you switch jobs you keep all unspent sklil points)

As for revocation, it is completely different, once you beat the game, you can revocate a character once it is at level 99 in a class. It resets that character to level 1 in that class, but you keep the skill points you have earned. Revocating is the only way to max out all your skills since reaching level 99 in all classes does not give you enough skill points to do so.

Revocating also has another benefit: it improves the chances of getting a higher quality treasure map when you complete a dungeon.