OP I'd pretty much guessed who you worked for from the onset given that we've both been paid on the same day
I've now got just over five years of service and while I'm not customer facing (I don't work in a shop) but I can relate to the issues your experiencing having seen some of them first hand. The "managers" from my experience are nothing more than whipping boys for the higher echelons - to be fair I appreciate they're sometimes stuck between a rock and a hard place receiving conflict both from their bosses and their own crews. But this doesn't seem to stop the majority of them from trying to lead through intimidation and bullying - to my mind a manager should have the ability to think for himself (or herself) and not just blindly follow procedure stating that's company policy which from my experience is predominately what happens within this company!
In aid to your specific case - though you're still within your probationary period you do still have rights. Regardless of the argument of pro's and cons, join the union and have a rep sit in with you in
any meeting you have with a manager. Even if for no other reason than you've got a witness to back up and clarify what has been said. Also keep note of every hour you work, clocking in and clocking out times, as well as any breaks and ensure it matches with what you've been paid. If you're not getting any joy with your immediate manager don't hesitate in going above his head.
Personally I
never ever do over time - I know they can easily pick up the phone and have a dozen foreign workers in to pick up the slack and meet the volume. This also means I know exaclty the hours and shifts I'm working each week and can easily define my monthly income as it should never change. Hope you manage to get this sorted OP - let us know the outcome please?