What I have heard (another rumour of course) is that ALL eligible cadets will be given mainline seniority numbers (based on previous seniority) before any are placed into mainline. So a senior Qlink Captain placed into mainline in December this year wouldn't be junior to a junior Qlink FO placed into mainline in August. Again, another rumour, but seems to be a fair system to me (I'm not a cadet).
Seems like a very sensible solution to keep all parties happy, which makes the rumour seem unlikely
Thanks for the info Di_Vosh!