The AFA integrated DOH because it is mandated by their Constitution and By-Laws, and there are hundreds if not thousands of labor organizations that do the same across the country. It won't be a comparison to an arbitrated list, it will be a comparison to the incredible body of seniority integrations that have take place. I.E. wide range of reasonableness. Your flight attendants had no say in the integration, it was simply a part of being a member. That constitutional mandate has never been successfully challenged because of the merits of longevity and the broad definition of 'good faith." I am sure many of your flight attendants felt disadvantaged. The idea of fair is subjective and their will be a wide range of views on what constitutes fair, however a strong standard exists amongst organized labor and its been legally upheld, time and again.
If you sue again, which is your right, the case will resolve around all the competing claims, the unions desire to balance those and DOH, especially with significant protective conditions to ameliorate the impact of the integration and whether that will fall within that wide range. You bring up arbitration because the largest pilot union has always taken the easy way out but even then, they have continually tinkered with it, vacillating on the makeup and at the whim of their larger members. A near plurality of other unions do not do so but their behavior has been found to be within the "good faith", standard.
The day the merger was announced, your flight attendants knew how it was going down before the first meeting. Good faith only requires consideration of all issues and a good faith effort to try and balance everyone's interest. Longevity has always met that standard when it has been challenged and coupled with protection through fencing, and as I understand it, one-way fencing seeks to balance the vested seniority of members and accommodating a disparagement in average longevity. Two people in the room has nothing to do with it. When they negotiate your contract, the committee that handles that doesn't requires a member from every position or seniority range in order to arrive at an acceptable solution for the collective group. It is simply evaluated as a whole on its own merits.