It's worth noting that this is NOT a contract, anyone accepted by EK will have completed all of the EK recruitment process, no different to anyone else.
So what if the guys decide to stay after 3 years, of course that they should keep their seniority number, after all they have given 3 years of service already.
The fact that they are there on a LWOP basis is really something in the background, if they choose to return to QF so be it, and the guys behind will move up a seniority number.
I think the realistic number will be 15 - 30.