I'd preffer the White Scars, both because I'm a fan, they are under represented and do fit in quite a few major respects.
Chances are that any given chapter of unknown succession are Ultramarines. But I'd like to think that the Storm Wardens are a special enough case to buck that trend if need be.
As you said they might be Imperial Fists, it might be that later foundings aren't missing the organs and it's only the first founding chapters (who basically have identical geneseed) and ones succeeding from it soon after that delveloped the problem later, or maybe it's one of the successful experiments to get rid of the flaws, perhaps even mixing it with other chapters seed. A White Scars / Imperial Fist mix perhaps?
It's only when rumors abound about them being derived from Traitor geneseed that it smacks a little closely of Blood Ravens for me (which might be unfounded in either case).
I've got no problem with them not knowing about their progenitor chapter, beyond the first founding this only really amounts to a label on a crate of geneseed. But I'd have more respect for a team of writers if they did buck this trend.