I am not in a position to address your "subtle rozdíl" question at this point, because I am a learner, too. But I can tell you that docela is accepted in the reverse exercise. The Czech sentence is the starting point for all exercises, and the author chose to use dost.
It can, in some context, but not where "special" is supposed to sound positive. I certainly wouldn't use it here to mean "special". If I wanted to say "The evening was special", I would use words like "výjimečný" (exceptional), "neobyčejný" (unusual), or "mimořádný" (extraordinary). Even the loanword "speciální" does not work here to mean "special".