No, it's not valid...smartass. It's ACCEPTED because two generations of idiots have been fucking up the English language, but it's not valid. It's the conjunctive/subjunctive and a conditional. If it were, not if it was. You could use "it was" and not "it were" but only if it WEREn't dependent upon IF. IF makes all the difference. There's no wiggling out of this one. You got egg on your face, now don't make a scene and just move on.
Also, just FYI...that/which is a completely different argument and has no bearing whatsoever on your mistake.