Is there a way to sally the garrison and then get them to abandon the province without fighting, so as to evacuate the area and hand over undisputed ownership (siege situation means the province is in a 'contested' state and belongs to no-one, neither faction gaining any income from it that turn)?

As far as I know, an unsupported sally only gets the pre-battle-menu option of returning back to the castle. Is that right? (Not encountered this situation myself yet).

It's another situation entirely if you start the battle, get such poor visibility (rain/fog) that you can't hope to find, let alone root out every last defender from those woods and you can decide to abort at that point but the retreat button will, I presume just send you back to the castle, with no option of abandoning the province.