The seniority list will go to arbitration. Which means, by definition, after the JCBA is ratified.

Think about it: If it did not go to arbitration, and the final list made someone upset...(imagine that), who would that group of pilots blame? Their own merger committee/union for agreeing to it.

Even if, by some miracle, both sides agreed on a list, by going to arbitration, each side can say, "We tried our best", but in the end a neutral decided and we have to live with it. And the responsibility can be shifted, at least somewhat.