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).
I think this scenario is highly unlikely. Reserved mainline seniority numbers for guys not yet employed in mainline was tried before (LOA161) and led to a lot of anguish. The policy was eventually thrown out a few EBA's ago. There is very little appetite amongst the company or AIPA to go down that path again.
Expect your seniority number to be directly linked to your start date.
If QF and QL can agree to release the cadets in their previous order then that would be one way of controlling their resultant seniority numbers. I don't see any other way allowed for in the EBA.