What Phileas says is also my understanding. What time you get to the hotel or go off duty doesn't affect your FDP, but it does affect how much rest you need. For instance if you have done a 15 Hr duty you need 15 Hrs rest, though your Flight Duty may only have been a couple of hours. There is nothing 'illegal' about this, but you do have to have enough rest.
You cannot be asked to go into discretion, or rostered into it and our original rosters do not ever do this, Unfortunately as the roster are about as accurate musket mistakes happen constantly. A culture has seem to have been built up where crews will change their on duty times in order to not have to file discretion reports, so very few get filled in. I don't like that one at all.