This kind of question comes up from time to time and I have yet to come across an NCAT (or CTTT) ruling that would support the right of an individual lot owner to acquire common property without compensation to the strata scheme and agreement to maintain and repair it.
But is the owner actually exercising a right to exclusive use. Or is he just using common property in a way that is permitted by by-laws.
The point I’m trying to make here is that it may not be the “land grab” that’s the problem but the behaviour of the land grabber.
So perhaps your appeal to the committee should be to stop the downstairs neighbour doing whatever it is that’s upsetting you, by looking at your scheme’s by-laws and then demanding that they enforce them.