A coworker is writing a sentence like
Sally, but especially Joe, enjoys questions about grammar and usage.
He thinks it should be
Sally, but especially Joe, enjoy questions about grammar and usage.
Leaving aside the possibility of rewriting the sentence, I want to understand which part of the subject controls the verb's number. Consider the following:
My parents, but especially my wife, [is/are] supportive of my goals.
My wife, but especially my parents, [is/are] supportive of my goals.
Dogs, but especially cats, [is/are] the cause of many allergies.
To my ear, are seems right in all three cases. If that's right, the rule would seem to be that if either part is plural, use the plural verb. I'm least confident about sentence two.
In any case, I'd like to learn more about what's going on here, grammatically. Is the phrase set off by commas a compound subject? An appositive? A subordinate clause?