If the merger is scuttled, you not only keep Horton, but Garton, Ream, and all the others who would have likely left. And a few who already had jobs lined up may still wind up leaving.
It's a good thing AA had all the managers and HDQ staff essentially re interview for their jobs as part of Cascade, and again as part of the merger prep. Now they'll know who they don't need to keep even though the merger isn't happening, and who to promote...
And yes, I don't see anything that says the company can't try to reopen S1113 as long as they haven't exited court supervision. It's not a one-trip salad bar, unfortunately. But it would have to be a pretty significant reason, and aside from the pension freeze vs. terminate, I'm not sure what else they could hope to gain. What they might do is look at some other contracts or leases, and perhaps put them up for cancellation...