Re the MCDU;
Don't worry too much - the basics of flying, instrument scan, and understanding and controlling the aircraft need to be acquired first.
But as I said earlier, if you have spare capacity it would be helpful if you know how to input a basic set-up e.g.:
City pair.
SID.
STAR.
Runways and approach type.
Cruise Alt.
Take off mass and V speeds.
Tower wind and outside air temperature.
How select and programme a holding pattern.
All these data will be given to you initially; you just need to know how and where to put them in via the MCDU - it is not difficult, but it helps to know.
If you can do these basic inputs, it will save time in the SIM setting up. It will also let the TRE see that you are reasonably competent and don't need baby sitting over every detail, which will make his/her life easier. But again, learning to control and fly the aircraft is the priority.