im not sure who else did (though i do think someone else did), im sure flynn said something about not getting back with leo, or it's a mistake, or something.
as for leo - i don't think chase wanted to, but he had to, for survival. leo didn't deserve to be left, but the group panics for a second once they realize leo didnt make it onto the train, flynn shouting fuck, for example. they didn't want him to be left back, but since leo left, with no one knowing, they didn't have a chance to bring him back. it was a matter of survival for everyone, leo just so happened to have gone through a traumatic episode in the small second, and that trumped his adrenaline to get onto the train.
there should have been a way to contact leo, but then again, we've already seen chase and how he acts when he's in an awkward situation, he becomes silent, as he did with leo when they broke up. he was young, and he uses that excuse frequently, because he knows he doesn't know how to handle it otherwise.
i do agree that chase should have said something else, but i can see why he said it! he was under a lot of pressure, pain, adrenaline and stress. he didn't have time to think, and probably said whatever was first on his mind: yes, i'll stay with you in echo, or no, i've voiced my opinion on echo.
if chase did decide to stay, leo's obsession with him was already getting out of hand, especially with the hysteria the town was going through, leo would have eventually gone to hurt chase, accidentally or 'on purpose' (because of the insanity). normally leo wouldn't, i agree with that, but the many factors along with no one to stop him, leo wouldn't have had anyone to stop him from doing whatever he wants with chase. he 100% had real love for chase, but the obsession would overrule his love.