As you can see I'm a Delta guy and don't have a dog in this hunt. What I will tell you from experience is that once it goes to arbitration all bets are off. Here are some short shots from the Delta/NWA merger:
1. It didn't matter that the majority of Delta aircraft paid more than NWA they just broke it down as widebody and narrowbody.
2. The did a stovepipe (which means they ordered the seniority list as to what you could actually hold so it didn't matter what seat you were actually in)
3. They merged the two lists (within a percent or 2 depending on where you sat on the list) so that ended up with about your same relative seniority.
4. They didn't care if you had a bunch of dinosaur aircraft that were approaching the end of their cycle life or if your seniority list had a huge number of guys retire early.
So basically they just looked at what type of seats you brought to the dance and merged them via a ratio by your size. We didn't have any furloughee's, however if we had they wouldn't have had a seat to merge into the list and as such would have been on the bottom.
The arbitrators didn't care about your date of hire as we had new hires (07) merged with 01 hires that had been furloughed.
Just remember you are now along for the ride since you signed off on the joint contract.