Off the top of my head, Sutcliffe and Kallis. There may be others, I don't know. It doesn't prove anything though. Clearly the fact that you're a great player increases your chances of making big scores. It doesn't follow, however, that because your highest score happens to be 189no that you're a lesser player than one with a highest score of 200.
1) I don't consider 50 to be a touchstone for greatness. Plenty of recent players average over 50 who wouldn't be considered "great". If you're looking for a numerical touchstone for greatness, which I think is a foolish aim in the first place, you'd look to set the bar nearer to the mid-50s for a modern player.
(2) A batting average is however a measure of a player's performance and ability over the course of his career. It's a far more reliable guide in those respects than whether your top score happens to be 189no or 200.
No, that's a false comparison.
Bradman has 12 double hundreds (or better). You are however talking about a bright dividing line between players with no double hundreds, and those with one. As you put it, "all great batsman have made at least one, except Sutcliffe". Your argument therefore appears to be that Kallis needs to tick the "double hundred" box on his CV by scoring one. And for the reasons I've given, I don't think any weight should be given to scoring a one-off double hundred.
If, however, you were to say that Bradman is a greater player than Kallis because, among other things, he scored 12 double hundreds to Kallis' none, then I'd agree with you.