I had actually just gotten blindsided by Chrom's "expiration date" last night, so I was wondering about this. His support rate was highest with my MU (my My Unit...!?), but I held off on committing them to the S-Rank, since I couldn't decide if I really wanted that pairing. But, then... turns out it happened anyway.
My only concern was that my hesitation had suckered myself out of their S-Rank conversation. Truth be told, I have an alternate save from many, many hours back where I went through with their S-Rank., so I've at least seen it. But I've got a question here: is there a support library in this game, as in past titles? And if so, is my alternate save enough for Chrom and MU's S-Rank to be 'archived'?