I guess the difference is CC at some of the
LCCs and Charter might only get a day or two
service training and 3-4 weeks SEP and AvMed training.
Have you seen how much stuff is crammed into those trollies. A lot of the charters carry more boxes of stuff in the hold for the return.
All airlines 'rob' parts from other aircraft to try and maintain
schedule integrity especially if you don't have a replacement part immediately to hand and say the other aircraft is on a longer turnaround
allowing more time to get the new part.
Why not just swap to the serviceable aircraft ? Could be for a variety of reasons:- different engine ratings, different config of fuel tanks (some aircraft same type have additional tanks) , different configurations ! All airlines do it and as long as done correctly, perfect safe.
Two out of the three airlines you prefer I know do it regularly - I have no personal experience of FR.