Your entire post is full of "Wat!?!" but this stands out. Your opinion is so full of suppositions I don't even know where to start. Are you seeking to apply the McCaskill-Bond act to this situation? Don't forget it works both ways and the Act has never had to deal with an arbitrated seniority award before, it is unprecedented and in all likelihood the 824 would remain unchanged.

Currently airlines facing SLI post-Act are using fences, DOH and blending techniques to get SLI to work. Like UAL/CAL. Not perfect, but the Act was designed to prevent Staple And Furlough thanks to AA's SOP of treating acquisitions like crap, in this case the precipitating event was the TWA buyout.

They can spout off "based on expected career progression" all they want, but there is abso-f'n-lutely no way a 2 year FO in upgrade class is going to leap ahead of a 10 year FO 25 numbers away from upgrade at a stagnating airline. This has not happened at any airline post-Act. This is why SLI's are quietly done by the company and unions during negotiations, and they expressly try to prevent situations like this because neither want to be mired in lawsuits and grievances for the next decade.

If anything, each respective regional would hold on to their own flow and be awarded flow opportunities based on their previous seniority list (unless contractually provided, then it's all up for negotiation in a new CBA) while bidding and combined list seniority would have fences and/or some sort of blending as needed.