What are you doing now?
I would take the job and get some flying done.
There's no guarantee about what will happen with the "mother" airline. Any paid flying you get is a bonus and I am sure you'll enjoy it. As you say, the waiting time you've quoted is "expected". That can either blow out to 3-4 years or shorten to 6 months. I would play the sure thing rather than risk missing out.
Yes, 1 years seniority may have serious ramifications but who's to say you'll stay with them for that period of time?
You'll have 500-1,000 hours in the log book which will make you more interesting to other companies should you choose to move.