I don´t remember if this scenario was ever explained but:

• In order to prevent blindness from Mangekyo Sharingan, one must implant the Mangekyo from another MS Uchiha user.

This leaves me to question, what happens if Uchiha1 with a completely blinded MS wants to transplant the completely blinded MS of Uchiha2? Will the vision of Uchiha1 be completely restore?

I think it was somewhat explained. Yeah, it seems like regardless if the MS of Uchiha2 has gone completely blind or not, the eyesight of Uchiha1 will be restored upon their transplantation. But with this condition of the Uchiha2 eyes' compatibility with Uchiha1 (close blood relative, ideally a sibling, you know). This was proven via Itachi's eyes transplantation to Sasuke - Itachi became blind (or almost blind) before dying, yet his eyes completely restored Sasuke's sight when transplanted to him.

Yes, like Ravenlot 27 said when eyes are transpalented they restore eye. Also when Sharingan evolve, if eye is blind, it is restored. For example; in Madara's case when his right blind eye(which izanagi is used) healed when his eyes evolve to Rinnegan.

Well, I'm not sure that any Sharingan evolution can restore lost eyesight. At least, this is the case of EMS > Rinnegan evolution.

If the condition to restore eyesight to a Mangekyo Sharingan is that it needs to swap owners, the Rinnegan evolution can simply be explained that way.

After all, Madara awoke Rinnegan because he recreated Hagoromo's chakra, which can also be interpreted as his chakra changing to Hagoromos, that is, a change in person. So the Rinnegan evolution restores sight because recreating these conditions essentially swaps his Sharingan from his body to Hagoromo's body.

As to blindness, the Mangekyo and the Eternal Mangekyo, I think Obito clearly proves that a normal Mangekyo is not supposed to lose it's light. Mangekyo losing their light is a weakness, not an intended feature. An EMS is not subject to this because whatever flaw causes blindness is fixed when two sufficiently similar (But different) chakras merge.

As to why this even fixes it, i would say that the donor's chakra stays in the eye, and is reinforced with the acceptor's chakra. This mixture is strong enough to resist wear. On the other hand, Obito's zetsu body was already strong enough to resist wear on it's own.

Could Susanoo be the main reason for becoming blind (note that i said main, not only)? It can also explain Obito's case. What i mean is that set of abilities (besides susanoo) mangekyo user gains are less taxing on the eye than susanoo itself. Just developing process to armored susanoo put a lot of stress on body and thus on eyes, and if u use it a few more times a vision became affected ...

I think you have a point here, but rather because of a particular reason. Susanoo appears to be the most exhausting technique in the MS arsenal - other ones like Amaterasu or Kamui are oneshot-technuqies who drain the necessary amount of chakra only once, yet Susanoo is a kind of technique which needs to be maintained via the user's chakra all the time. I mean, it's constantly exhausting the user and his eyes. If there would be a limit to chakra that can be used on maintaing the Susanoo, the technique would affect the user not more heavily than any other MS technique, but since it can be maintained until your chakra reserves are almost depleteled, the amount of strain on user's eyes and the whole body is much more powerfull than the one from any other MS jutsus.

Itachi's Mangekyō did last for quite some time as well, though not as long as Obito's. I agree on the Zetsu part (though I'd say it was Hashirama's cells which helped) slowing down/stopping the blindness induced from the dōjutsu's overusage.

Even the theory about Susanoo being the main reason for inducing blindness is good.

https://www.reddit.com/r/Naruto/comments/2mi8cr/translation_completed_uchiha_sasuke_4th_databook/

"イタチの命と引き換えに、サスケは直巴の〝万華鏡写輪眼” を開眼。" = "In exchange for Itachi’s life, “Chokutomo”/Straight tomoe of the Mangekyou Sharingan were awakened."

"移植されたイタチの目は "万華鏡写輪眼" に永遠の光を灯し、 兄をも凌ぐ忍術と瞳術をサスケに与えたのだっ た。" = "Implanting Itachi’s eyes, the Mangekyo Sharingan, it gave him an eternal light that was turned on. It also surpassed his older brother’s ninjutsu and doujutsu." From OrganicDinosaur herself. Eternal Mangekyo Sharingan IS the Choku Tomoe, hence only Madara and Sasuke have it.

Wait. This passage could also be interpreted as "Because of Itachi's death, Sasuke awakened the Choku Tomoe Mangekyō Sharingan".

Though I agree that Choku Tomoe=Eternal Mangekyō Sharingan.