FYI - PowerPrep is an old piece of software that does not reflect the actual scoring of the GMAT. GMATPrep does - it was built using the same algorithm.
You list your first category as "verbal" but then say you missed 4 "quant" questions in that section. I'll assume you meant to say verbal there; correct me if I'm wrong.
On verbal, the score pretty much tops out around 46-47 (99th percentile). I know the chart goes up to 51, but you really don't see that. That's just FYI, though - PowerPrep isn't going to accurately reflect the real test because it's not built on the same algorithm.
Re: additional quant questions at a higher level, you can try GMAT Focus (
www.gmatfocus.com). These are official questions but they're available online only, in the form of a diagnostic series of questions. It's adaptive, but you can't select certain areas - you just take whatever the algorithm gives you. There should be a number of number properties problems though, just given the nature of the test.
Combinatorics and probability are really not very common; most people see just one or two of these (total, for the two categories combined) on the entire test, so I would rank these areas much lower in terms of priorities. Most people also see only one or two set problems - so *slightly* more common than comb & prob, but not much more so.
If you really want to try finding some very hard questions, you can also try looking through our Challenge Problem Archive, but I should warn you that a lot of those questions are MUCH harder and more computation intensive than what shows up on this test. But I remember some of your earlier posts and know you are going for a much higher goal than most, so perhaps these will be useful for you.