E.g. "as if she were the queen of England" my readers almost uniformly change this to "as if she was the queen of England." (And sometimes "Queen")

"If he were there, he would kill Darth Vader."
"If he was there, he would kill Darth Vader."

Almost everyone stumbles on my use of "were" so am I following the wrong rule? How is this handled in most mainstream fiction?

The subjunctive form is used to express "uncertainty," i.e. wish, possibility, opinion.

From your example, my understanding is that the correct way of saying it would be "If she were the Queen of England." I would use "were" instead of "was" because the subjunctive form is expressing a possibility or an opinion of what a person would do, not what they will do. Also, I would capitalize Queen because it is part of the queen's full title/name (you're using it like a proper noun in that sentence).

I'm sure there are grammarians on the forums that could explain this a lot better than me

Edited to address your question as to how to handle it in mainstream fiction. My personal opinion is that just because it's mainstream the grammar shouldn't suffer. I think it's OK to break the rules if you are trying to make it the point. That is, you are using "bad grammar" to denote someone's speech, way of thinking etc.

PS: EricJamesStone, your succinct and witty reply gave me a good chuckle.

Most readers aren't going to question your grammar. Beta readers are reading specifically to find faults, most readers are just reading for a good story.

If someone does question your grammar, maybe they will look it up and learn something.

And yeah, I've had betas 'correct' me on it too. And on a number of other grammatical points that evidently haven't been taught in recent decades, since it seems to be exclusively an under-30YO problem.

If you need an editor, either find a good friend who likes editing so much he/she will be willing to look at your work for free, or just hire your own (if you're planning to indie publish your own work this is a step you'll have to figure out at some point anyway.) For some, if they see what a professional editor adjusts just once or twice they can modify on their own and eliminate many of the common mistakes editors catch. Others it's worth it to pay for an editor to read and really find the errors.

But for beta readers, in my opinion, they shouldn't be looking at verb forms and other wordsmithing persnickety things. Does the story work? Are the characters believable? Did they root for the right people? Were there any major plot holes or mistakes (I am notorious for changing my mind about a sub-character's name about 1/3 of the way through my novels, for ex.) Are there places where they found their interest waning? Did they stop believing in/rooting for the main character at any point? Would they recommend the book to a friend? Did the ending make sense? was it foreshadowed enough, too much, not enough?

Those sorts of things should keep them plenty busy that they don't have time to critique verbs that are used correctly.

Actually, it's peculiar that inexperienced struggling writers and critiquers use linear revision processes and strategies, beginning with lexical and syntatical unit emphases, and overlook larger content and organizational unit recursive revision processes and strategies. Which experienced struggling writers and critiquers practice first then delve into lexical and syntatical revision processes and strategies as an outcome-end process and strategy. Recursive methods perceive a multidimensional architecture. Linear methods perceive a two-dimensional structure.

Sometimes I have readers who circle every time I use Affect/Effect and put the other one. I am nearly always right, they just have been turned around by teachers who tell them they are usually used wrong. Same thing once in a while with whom, then I have to have the Direct Object talk. (Although I'm ashamed to admit I have to look up lay/lie ever stinking time, which is why I avoid them usually.)