ALPA merger policy would protect furloughed pilots if there was a merger with pilots on furlough. Its not saying those pilots would come back quickly, especially if it was Frontier or an LCC because those airlines aren't really growing and it would take a long time for jobs to appear. The biggest problem is that while a smaller Spirit is a better "bite sized" acquisition, it still has all the debt associated with it. Effectively making it a poison pill acquisition.

WRT the "pilots must go with planes stratification clause" your CBA can't force another airline to take pilots just because they bought your planes. Other airlines are not a party to your CBA. Those clauses are generally only feel good clauses, and usually say the Company must make a good faith effort to negotiate jobs at the acquiring airline. But they aren't going cancel a plane sale because the other airline doesn't want to integrate a bunch of pilots that it didn't get a chance to interview and make a hiring decision on. The acquiring pilot group would be largely against this as well since its possible the acquired pilot group could try and claim an SLI under Fragmentation policy, which has never been formally implemented before.

Only if there was an acquisition or merger would pilots actually "go with the planes".