I don't know what the solution is, but if I was being treated that way I would find some way to do what I wanted without their permission & tell them to go fuck themselves. Maybe all of the land can be put into a trust or sold to a corp you create & divided via some private contract or like as preferred stock which can be privately exchanged...?
Discussion
The FIL can build an accessory building for a “farmhand” but then we won’t have a separately deeded lot, which is what we all want. Just thinking ahead for the future if properties are sold, they can be sold separately. It just shouldn’t be this hard to severe off a lot from your own property when it does infact meet all requirements.