Actually EMS is the perfect Sharingan.

Hence the name Eternal Mangekyo. You receive all the powers previous forms had without any drawbacks. And what does Senju DNA has to do with anything? You get EMS by transplanting eyes, so yeah it's not pure, but it's still withing one's bloodline so it can pass as a legit Sharingan.
This however, doesn't go for Rinengan, RinneSharingan etc. because they use external bloodlines for the sake of unlocking them. And yes you are. A forbidden technique is indeed a technique harmful to the user, that's it's definition. You basically said "it's harmful because it's harmful". We know that. But how does it do that, as in harm the user? That's the question. Answer: huge drain of chakra and stamina as well as damage to the optical nerves.
And no, it being forbbiden doesn't mean you weren't meant to have it. It just means that it's viewed as socially unacceptable. Thus causing those who use them to be looked down on in the society. It's just the norm not to use it. Awakening it is completely natural, well as natural as a deep emotional trauma can be.
And yes, ofcourse MS Sasuke is stronger than the Hebi Sasuke, but it's not due to increased chakra, but the unlocked abilities. As for how can you use techniques like Susanoo? Easy. Unlocking condition doesn't mean you can be a profound user. You just need the eyes to unlock it, but sustaining it is a nightmare. That's why all the MS users broke down soon after activating it. Just look at Itachi and Sasuke. The guy broke down after only activating the rib cage.
And Sasuke indeed has huge chakra reserves. That's how he was able to keep it so long in his fight against Danzo. However, every Sasuke had huge chakra resrves, since he is Indra's descendant. The reason why we see him lasting longer and longer could be due to the fact that, just like everything else, he trained enough to tap into it more and more.