I don't quite get the part with the money: Harry used his foreknowledge of the future to know which companies would do well and where to invest? Hmmm. Just because he went back ten years in time doesn't mean he actually knows that. I mean if I suddenly found myself back in time ten years, I'd be absolutely clueless as to where to invest!!! And the Harry we see in the books never has anything to do with money or investing - other than him gifting the twins with his Triwizard winnings for their shop. Did Harry check out the magical stock market for the past ten years just before going back in time or something?
Well, it just isn't logical to me. (I even dissect JKR's books logically at times and end up very disappointed due to some holes in her logic you could drive a truck through! lol ;))
Not the magical stock market (which is unlikely to exist) but the Muggle. As I said in the chapter, Harry often heard Vernon ranting abut how much much he would have made if he had been able to invest on the groudn floor, etc. etc. etc, and Harry based his investments on those rants.
Something of a cliche, but not nearly as improbable as you suggest