I think this would open up the possibility of cyclical time in G2. Takanuva becomes new Ekimu, cue new toa, seventh toa, then seventh toa becomes new Ekimu. Wash, rinse, and repeat.
Umarak puts on the MoCo, and his individual will is lost in the mask's/Makuta's. It's the mask that controls the wearer, and not the other way around. I guess that makes the MoCo a quasi-version of the One Ring.
I don't know what bearing, if any, that protector statue has on the possibility of a third mask maker/ unknown mask-making entity. Okoto clearly has a long history, and the presence of one statue (of a character we are already aware of) in the middle of the desert does not indicate anything other than lackluster worldbuilding on Lego's part.
Another misconception that could be addressed is the idea that Mata Nui and Metru Nui are two different islands. Actually, this is something I don't fully understand either. I guess one's above the other or something?