I can totally see Roger and Mel dating for approximately a week and a half, at the end of which Roger decided her cute did not make up for all the feminist arguing. Maybe in his fourth/her third year? He's not one to view finishing with a girl as a big deal, so he'll probably be just as friendly if they dated and broke up as if they'd never dated at all.
Roger actually quite enjoys trash-talking, so Lee stanning for the Gryffindor team wouldn't be a problem with him at all. I'd imagine the two of them could be casual friends.
He is way too much, but Roger has enormous respect for his fellow Quidditch players, so he'll try his best to be nice.
Yeah, the only thoughts Roger are likely to spare for Yakov are along the lines of, "lol, it's that messy Russian kid."