Should co-first authors be listed in alphabetical order?
I don't see any way to view this other than as a misunderstanding or miscommunication. (Not necessarily an innocuous misunderstanding: perhaps someone is trying to pull a fast one of some sort.)
I am a mathematician, so maybe I am especially sensitive to logical issues, but I can't think of a situation in which someone tells me something that sounds like a logical contradiction in which I wouldn't just say, "I'm sorry: I must be confused because that sounds contradictory. Could you please explain it again?"
My only guess at the resolution of the contradiction is that someone is in fact trying to change their mind about the ordering of the authors. I know little about the conventions and nuances of author ordering (because in pure mathematics the order is almost always strictly alphabetical), but I do think I understand the meanings of all the terms involved, so I can only imagine that if you submit a paper to a journal saying "by Bravo, Alpha, Charlie....Please note that Bravo and Alpha are co-first authors" then the editors will respond by calling attention to the contradiction. What else?
When talking about Grad Student B, you said
if it were not to him, there wouldn't be a paper.
If that's not a good definition of "most important contributor", I don't know what is. B should be the first author. If you want to make sure that A gets due credit, and the journal in question allows this, I would suggest adding a footnote specifying who did what (I typically ask my undergrads to do this when they write joint term papers).
Should co-first authors be listed in alphabetical order?
How is being co-first authors supposed to work in general? I come from a field where authors are typically ordered by contribution, but, by definition, there has to be exactly one first author. Being the first author of a paper is not some honorary title that can be split - it is simply the first gal/guy on the author list. Hence, by convention in my field, this is also the gal/guy that the authors have agreed upon has contributed most to the paper and should receive most recognition.
What is important here is that just because "A" is the second author of the paper, it does not imply that he did not do anything substantial. It merely means that the authors have agreed that her/his intellectual and technical contribution was at least a tiny bit less central to the paper as "B"s.
Do you think A would be entitled to feel some kind of injustice at not having his name listed in alphabetical order? Is there a consensus that co-first authors should be in alphabetical order and is the editor likely to point this out?
No, "A" has no right to require alphabetical ordering. The editor will not care, because how would he even know that "B" was supposed to be a co-first author?
The more important question here is whether "A" has a right to feel injustice because she/he in fact believes that her/his contribution was in fact larger than "B"s, hence, that "A" should be the first author. This is a question that we cannot answer. Note that the number of figures contributed per author does not seem extremely important. Likewise, who ultimately submits the paper is not the deciding factor. That "B" has spent multiple years working on the project sounds like she/he had in fact a lot of impact on the work, though (assuming that she/he did in fact not just idle around and procrastinate, which we cannot tell of course).