You “plan to attend” or “plan on attending.” If you were using something more abstract, I’d use “for” — that is, “I didn’t plan for that.”

As to your question about quotation marks, this is often quite confusing. Generally I put end punctuation inside of the quotation marks. I say generally because there is always an exception, particularly if the quoted material is a the end of a grammatical sentence but the quote does not contain a complete sentence. In this case, I have been known to place it outside, but it drives MS Word nuts.

Should he use “plan on” or “plan to”?

If the jury accepts these examples, would it also accept, as a general rule, “If a sentence concludes with a phrase isolated by quotation marks or parentheses, and that phrase requires terminal punctuation, such punctuation shall serve the entire sentence”?

When someone says that they “plan”, they usually mean that they “intend”. In that sense, “to” is clearly the way to go.

When is it accurate to use ‘plan on,’ and when is it accurate to use ‘plan to’?

Whereas we Americans:
When is it accurate to use “plan on,” and when is it accurate to use “plan to?”

I think “plan to” is actually the more correct of the two options given. “Plan on” seems to be more colloquial in usage. I’d say use “plan to” in writing and “plan on” in informal speech.

also, because the “?” is to go on the outside of your quotation marks because it is apart of your sentence and not the sentence you are quoting.