The answers to all your questions are "It depends" (except the first one which is "yes").
In your case representing your maximum drawdown as a percentage seems pretty sensible because your bet size is a proportion of your bank so your drawdown and your bets are in the same "units". Although, if you're reducing the size of your bets as your bank shrinks who cares what the maximum drawdown is - you should be able to take a huge drawdown without exhausting your bank - until you get to the minimum bet size of your bookie. Of course, building it up again is going to take ages with the teeny weeny bets that your miniscule bank allows (not a fan of reducing bet systems myself).
I tend to increase my bet size in the (unlikely) event that my bank passes a certain size and then don't reduce the bet size. I suppose logic would suggest my my maximum drawdown should then be expressed as a number of bets. The logic being that if my bet size is 1/200th of my bank non reducing I can take a maximum loss of 200 units.
As for whether your 58% is acceptable, using jfc's drawdown calculator you will find that maximum drawdown increases (not surprisingly) with the number of bets you make. If you have lost 58% in two years of betting - how long do you expect to use this system? I'm guessing if you use it for 10 years you will probably experience a largest losing streak of at least twice as big as the one you've had in two years.
So the decision my friend is yours. Grasp the nettle with both hands, follow the piper into the cave and stop prevaricating. You only live twice - or so it seems.
KV