He might not turn out to be a great opening batsman, maybe even a very poor one. But he's certainly looking like an opener now, he seems to have adjusted his game very well to it.
I don't think 2 games is enough evidence to suggest that. He's opened well this Test, not disputing that, but this so far is the whole 2nd time he's done such a thing.
As i've said before, i've never seen a century opening stand with no play+misses, edges that didn't carry, dropped chances etc. Smith was very lucky with the inside edge nobody appealed for. As for McKenzie's lbw, his bat was close enough to the pad to make the umpire's decision fair, if not at all the correct one. But i don't think those incidents are so damning that you should feel the need to qualify every statement you make on the quality of their innings...
Smith should've been out - simple as that. Even though no-one appealed, it was a nick that was caught.
Play-and-misses and balls landing just short of or out of reach of fielders, as I've said umpteen times before, are totally different to dropped catches, missed stumpings, Umpiring repreives, etc. You'll be waiting a long time if you're searching a century \ century partnership without one of them. But let-offs are something that will usually result in dismissal. So normally, you edge one to the wicketkeeper, you're gone. If you need one of those every century, you'll not get that many.
Basically, though, it's rather imprudent for me not to mention it. I always do, and some people like to pretend to themselves that I mention it loads for players I "like" and never for those I "don't like". Which, of course, is nonsense, so I need to demonstrate that.