Subject-Verb Agreement – Plurality in ‘A Second Set of Motors [Is/Are] Attached There’

Were is absolutely correct here. The reason that it's were and not was is that it's a compound subject (which is indicated by and).

Your editor has got a sharp eye. It's definitely awkward, but it's technically correct, which is why you should, as you've suggested, simply re-write the sentence so that the singular item is not the first item in the list (this obviously won't work though if you have all singular items in your list, which I'll explain in a bit).

English grammar does not dictate--to this native speaker's recollection--that the first item in a list indicates the grammatical number. I will say, however, that there were an orange, grapes, two apples ... sounds awful to my ear (and I suspect that this is true with most speakers).

I also think that most speakers make this error without realizing it with expletive constructions (i.e. sentences that are constructed in the order of there/it + be + subject, rather than the more common, active voice, Subject-Verb-Object order). It's generally best to avoid expletive constructions whenever possible (it's not always possible to do so, but in your example, there is unnecessary)

The indefinite article an is ordinarily a marker of singularity, which makes it sound as if it should be was, but with compound subject, that's not the case. It still would be were even if it were**: There were an orange, grapes, an apple, and a cherry [ALL] piled on the plate. Or, even better: An orange, grapes, two apples, and a small pile of cherries were all on the plate.

If the sentence were**, There was an orange, and nothing more, than was would be correct. But because you're talking about multiple subjects that are/were grouped together, the subject is plural.

There are four objects in this list--that's counting the grapes and cherries each as a single unit; technically, there are more--so there's no question that it's plural. This isn't a one-or-the-other situation; it's collective.

A general rule of thumb would be that when you're using and as a conjunction, it's a compound/plural subject, and thus indicating a plural form of the verb. If instead you're using or, then it's singular, regardless of the article.

**I'm using were in two of the above sentences with a singular subjects because I'm using the subjunctive mood; otherwise it'd be was. That's the only time it's ever correct to use were with a singular subject.

From Garner's Modern American English Usage, 3rd Ed.:

Language-Change Index

• there is (or there's) with a compound subject whose first member is singular <there's an outhouse and a sump pump out back>: Stage 4

Key to the Language-Change Index:

Stage 4: The form becomes virtually universal but is opposed on cogent grounds by a few linguistic stalwarts (die-hard snoots).

Garner's Stage 4 Analogies:

School-Grade Analogy: Grade B

Golf Analogy: Bogey

Olfaction Analogy: Vaguely odorous

Skill-Level Analogy: Amateur

Military Analogy: General discharge

Etiquette Analogy: Elbows on table

Traffic-Penalty Analogy: Warning ticket

School-Discipline Analogy: 1-hour detention

Moral Analogy: Peccadillo

Parliamentary-Discipline Analogy: Warning

As you can see, this is not quite standard yet, but Garner notes that using there with a compound subject is widespread nevertheless. He notes that purists still object to it, and to my interpretation, that it's technically incorrect. Therefore, while it'd be perfectly acceptable in speech, it shouldn't be used that way formally. Very careful speakers/writer's will object to this usage in writing.

Diana Hacker And Nancy Sommers also indicate that singular compound subjects take plural verbs in expletive constructions in Rules for Writers. They agree that in sentences beginning with there be--i.e., when the subject follows the verb in a declarative statement--the subject is plural if it's a compound subject, even if the first item in the list is singular, and therefore, it's got to take a plural verb

From Rules for Writers, 7th Ed.:

21g Make the verb agree with its subject, even when the subject follows the verb.

Sentences beginning with there is or there are (or there was or there were) are inverted; the subject follows the verb.

There was were a social worker and a neighbor at the meeting.

The subject, worker and neighbor, is plural, so the verb that follows must be were

Occasionally you may decide to invert a sentence for a variety of effect. When you do so, check to make sure that your subject and verb agree.

Of particular concern is are penicillin and tetracycline, antibiotics.

The subject, penicillin and tetracycline, is plural, so the verb must be are.

From The Chicago Manual of Style Online, 16th Ed.:

5.32Exceptions regarding pronoun number

There are several refinements to the rules stated just above: (1) When two or more singular antecedents denote the same thing and are connected by and, the pronoun referring to the antecedents is singular {a lawyer and role model received her richly deserved recognition today}. (2) When two or more singular antecedents are connected by and and modified by each, every, or no, the pronoun referring to the antecedents is singular {every college and every university encourages its students to succeed}. (3) When two or more singular antecedents are connected by or, nor, either–or, or neither–nor, they are treated separately and referred to by a singular pronoun {neither the orange nor the peach smells as sweet as it should}. (4) When two or more antecedents of different numbers are connected by or or nor, the pronoun’s number agrees with that of the nearest (usually the last) antecedent; if possible, cast the sentence so that the plural antecedent comes last {neither the singer nor the dancers have asked for their paychecks}. (5) When two or more antecedents of different numbers are connected by and, they are usually referred to by a plural pronoun regardless of the nouns’ order {the horses and the mule kicked over their water trough}. (emphasis added)

People is the plural form of person (see irregular words), therefore, it has to be referred as plural. People are always minimum two, that's why we should say:

"There are people who like to help." [Correct ✅]

rather than

"There is people who like to help." [Incorrect❎]