Will it be worth it/extremely hard to learn C++ just to use Qt? (I always read here and reddit about how hard/inefficient/easy to fail is everything is in C++)
Worth it if you need to use a cross-platform desktop framework and don't like java. C++ is most definitely harder to use than many (newer, thus) more modern languages but you will be rewarded by the niceness of Qt.
I am not sure about that. Not in the sense of gaining wisdom as would be in case of, say, Lisp or Haskell -- or even C. C++ is all just nuts and bolts, a more or less coherently implemented collection of paradigms originated in other languages. It is very useful from the practical standpoint, no doubt, but being "worth knowing", "superficially" - not so much IMHO.
(Eng. background and software dev full time)