@stainsoftime A lot of your magic section doesn't work.
The Magic Winds do not come from the land. They come from another dimension entirely. They enter the land through, essentially, invisible tears in reality. The larger, and more, tears there are in the land, the higher the magic density would be. It isn't possible to 'drain them from the land' as Greed is doing. He can say his capital is built upon an especially large tear and he absorbs all that comes out or that he has artifacts designed to soak up the magic, but it isn't possible to just 'gobble' it all up [I mean.. I would probably let Gluttony do that.. Because... Gluttony..]
If his people have a high magic resistance, they cannot be used to 'store' magic. Being magic resistant means magic just doesn't 'hit' them the same, so if they were magic resistance, especially, incredibly high magic resistant, there is no way you could store magic in them. Would be like storing ice cubes in a volcano. Just doesn't really work. Also remember the more resistant the people, the less mages are born and the less 'talented' they are in using magic, so they'd never be capable of matching, say, Zodia [Lust's domain] where large number of mages can command 3 winds.