Taking your points in order:
I rate Keane higher too. I think he's got the potential to be a top class defender. But that doesn't mean I don't think Blackett is ready to be a back-up for us.
He only briefly played CB in the U18s, and as you say playing CB in a two-man central defence is effectively a different position to doing it in a three anyway. It's the latter that he looks particularly suited to. I'd never heard about any attitude issues so I can't comment on that.
Agree on Tom Thorpe, as a tradition two-man CB I rate him higher than Blackett, probably equally as highly as Keane. On paper he's arguably got the more convincing record than Keane. It's this quantity of promising youngsters in the position which makes me confident we'd be fine relying on them as back-up. You never know who might quail under the pressure of the PL, but if one of them does, there'll be two more ready and waiting to take his place.
I've been really impressed with Evans on tour. He was shite in the Liverpool game, but then so was most of the first-half team. Until that game he'd been excellent, particularly standing out against Madrid. That said, I do think his best position is at the centre of that 3-man defence. The central man is the one who can step out, interrupting attacks early, bringing the ball forward with freedom, and almost acting as a DM at times, and that's an obvious strength in his game.
My attitude stays the same, though. If there was a top-class left-footed CB out there, someone at the level of Hummels or Benatia (both righties), then I'd be all for it. But if we're looking at mid-range options who aren't guaranteed to do well themselves, like Vermaelen, I'd rather we just stuck with promoting the kids to support our existing three.