Try not to worry unduly about the catheter. It's usually not a problem for most men. You may be surprised how quickly your body adapts to the presence of a tube exiting your body and being connected to a floor bag or a leg bag.
Sleeping on your back may not be your normal sleeping position but the body seems to accommodate what needs to be done and gets on with it. I am not aware of any one losing their catheter during sleeping.
When it is removed, your urethra will need to get used to a new "closed" position, as it has been "closed" around a pipe for some weeks. The removal of the pipe may leave a gap that you may leak from? Your tube will need to get used to taking up the slack and shutting down completely again as it did pre op.
Looking back on the time I had it in situ, I was one of the rare occurrences of having problems with my catheter, then it was removed and I went to pads, 3 or 4 a day, but I would "bag up" in the evenings or when I got really tired. Eventually I did not need to bag up at night and did not even need a pad. 16 months down the line all is good. We all recover at our own rate.
You will be fine. Any problems post up here.
Dave