I'm working under the assumption that T3 isn't the solution for all cases of hypothyroidism. I haven't looked further into it but I can imagine if someone has an issue with low thyroid binding globulin, adding more T3 wouldn't necessarily help since the issue is with not enough transport proteins being produced by the liver (maybe a few other similar exceptions but I don't know for sure which is why I said depending on the reason).
You're actually already one of the more intellectual guys on here which is why I <3 you soooooooooo goddam much
Not 100% sure about your last question but considering the average thyroid produces about 25mcg of hormone and most people supplementing will be taking at least double that and generally even more, it's not hard to reach supra physiological levels.