Back to your original question.
So far, I've seen 3 different scenarios on seniority when two companies merge.
1) List is unified by Date of Hire. In my humble opinion, the fairest scenario, although inevitably people on both sides are unhappy (Captains returning to the right seat and the big fish in the small pond being fish in a big lake).
2) Winner takes all. The pilots of the "acquired" company go to the bottom of the seniority list. Losers very unhappy. This happened when American aquired TWA. Should not be a major factor, mind you, in the event of an EK/EY merger since seniority doesn't really decide who flies the left or right seat.
3) A 2:1 type of merging scenario-- where the pilots of the victor company get two seniority places for every one of the "losing" company. Again, people unhappy from both teams. This is what happened when Air Canada acquired Canadian Airlines.
Overall, combining workforces has never been a smooth an happy situation in any airline merger.