Looking at Micaiah, Cordelia isn't Mary Sue at all.
I felt like choking Micaiah in Part 3.
But she got Chrom's heart with the pies.
Back to topic
Between the two, I'd say Sumia.
Actually I kinda reluctant to pair anyone with Cordelia other than Chrom, which is impossible, it seems like some rebound I guess?
Especially after Severa said "Chrom this Chrom that" or something like that. It seems like even after had Severa, she still have some feelings for Chrom.
...tough my Fred wed Cherche.
Appearance-wise I'd say they look good together.
and he got to father Gerome.
And black-haired Gerome. Awesome.