My knowledge in this area is still very little, but I'd been researching as much as I could since I just found out I have hypothyroidism. My understanding is that the T4 is used because of the longer and more stable half-life and the conversion factor, rather than pouring T3 in, which would "replace" normal thyroid production.
Again, I could be off on that, but that's what I've come to understand it as.
As far as the amount of T3, I don't think you'll ever see one blanketed answer. The conflicting answers you've seen will always be that way. And I think a big part of that too, is the individual differences in terms of tolerance, and your normal thyroid production, etc.