Which is correct: "what if there was" or "what if there were"? [duplicate]

Prescriptively, you're correct, this should be were since this is being expressed with the subjunctive mood. Descriptively, I think you'll find both in the wild. In informal speech, most people I know would prefer was in this case (and those that don't are sticklers for the subjunctive). I do think that you are much more likely to see were written though, especially in formal writing. Either form is correct, and the subjunctive forms are far from dead.

A more interesting question, I think, is whether or not the subjunctive mood is still being used and if the two forms have simply collapsed together. Is this a meaningful distinction? How could this be tested?

"WERE" because it is the past subjunctive of "to be". It's an oversight by the copy-editor. It's not the biggest deal in the world since the verb "to be" is the only verb in Modern English wherein the difference is obvious, although, if one WANT to speak very formal, correct English, stick with the subjunctive.

Not a native speaker, but indeed it is a conditional and therefore "were" should be used. However, I hear the incorrect form more often than the correct one, so I guess that "was" is a colloquial, albeit grammatically incorrect form.