The skill is not possible without the sacrifice of an eye, this is the cost of the skill, I did read your post (maybe not meticulously as it is a comprehensive post) and I understand what you are saying.
The difference between MS techs and the two Iza's is that the blindness from MS is as a side effect from over use, whereas the blindness from Iza is the actual cost of using the jutsu.
Izanami seals away the Sharingan of that eye of the user forever.
Izanami and Izanagi are the most powerful techs of the Uchiha, as they bend reality itself, that's why they have the highest price.
You used Tobi in your example, but are you sure that Tobi's eyes are not transplants?
You used Itachi, but Itachi as an EDO was not going blind from using MS techs, yet when he used Izanami he lost an eye, this was becuase the eye was sealed[/B away, EDO's are not immune to Fuinjutsu type effects.