I don't exactly get Elwing either (but that's partially because I'm a bit - ok, much - of a Feanorian fangirl and thus I don't get all this "We mustn't let them gain a Silmaril under any circumstances" anyway) - but I can't help but feel impressed by her courage (etc. etc.).
Personally I assume that Elwing really had no idea whether or not her children were alive while she jumped - but neither could she be certain that they weren' alive, and yet she decided that keeping the Silmaril was more important than running off to look for the kidlets. Which must have been quite a hard choice. And later on - if she ever learns that the kids are alive, not entirely sure how forthcoming the Valar would be about that sort of information - she does give the Silmaril to Eärendil, as you say, even though it'd be perfectly conceivable that she'd rather insist on using it to ransom the twins back. Which she doesn't.
So yeah, on the whole - various decisions that I wouldn't have made, but that definitely must've taken a lot of strength and determination.