1. That happened way faster than I was expecting it too. Itachi managed to have his Mangekyou for over a decade without going completely blind. Sasuke is losing his sight after, like, he's second day using them. Does he just suck? Or did he use his Mangekyou powers as much in 3 fights as Itachi did in 10 years?
It's not too early. Sasuke's been relying on his MS powers since he got them.

Itachi was right up there in class already. He was a lot more efficient and smart about beating his opponents, leading to less use of his taxing powers. And when he did use them, he uses them for like a second or so, making the most use out of it with the least damage to himself.

Sasuke's been spamming it the whole time. I'm actually surprised it didn't blind him earlier. Right at the beginning of Shippuuden, Kakashi already noticed the blinding effects of the MS, and it sounded like he only played around with it for a little.