What would happen if the 1 CMC got their cutie mark, but the others didn't?

2242458
I suppose they'd celebrate, and the filly with the cutie mark would share tips with the others on how to get theirs, but the cutie-marked filly would still participate in their stunts. I'm pretty sure that by now, most of the things the Crusaders do are for the fun of it.

2243292 Don't you think the others may be envious or jealous?

2249309
Probably, but it's my opinion that they're too much good friends to let that get much in the way.