- Have you been practicing the song? These two version sound good to me. Were you practicing the song? sounds odd.
I have been practicing - an activity that continues in the general present time, not necessarily at the moment of speaking. You may be at the moment in Uruguay, for 3 days, and say 'I have been living in France since 1995'. On the other hand, you can tell your wife 'Please stop! You've been shouting at me for the past 30 minutes.' and imply she is still shouting right now...
the word exercise is very rarely used in this context in the UK. the word exercise was used a little when i was at school, many years ago- we had 'exercise books' in which we would write maths sums etc. however, to me it makes more sense to alwways stick with practice... e.g im practicing to speak english I didnt practice much english yesterday.. hope i havent created any confusion!
Practicing law/to practice law is the goal of most law students. In the OP sentence I think that only the noun "practice" may be used. "You need a lot of practice/lots of practice". "Lots of practicing" is used sometimes, but I would say that the two words are not interchangeable: It takes lots of practice/practicing to become expert at any sport.
If you are exchanging with a Canadian, and especially if this is a familiar dialogue, je me pratique (beaucoup), would be the most natural response. If you reply in conversation "je m'exerce", it is obvious that you have been practicing in the classroom and not in real life.