"Three types of phone" ("Three phone types" is also grammatically correct, but less usual).
"The doctors' cars" ("The cars of the doctors" is also grammatically correct, but clumsy).
There are a few issues that seem to be in play.
In most cases, a reference to plurals in rules also includes the singular. And vice versa.
In the event of errors, accommodation will be made.
While the plural is used, most readers would believe that this applied to a single error as well as multiple errors. Similarly
In the event of error, accommodation will be made.
It means the same thing.
Sometimes, to make clear that both singular and plural circumstances apply, parenthetical plurals are used.
In the event of error(s), accommodation will be made.
This makes it explicit that both singular and plural problems will be adjusted for.
I am not aware of a term that names this characteristic (or conundrum).
As you have suggested, this is a problem often faced in legal drafting (which you wish to avoid). To ensure clarity, many contracts say something that means "plural words can cover singular events and singular words can cover plural events."
To convey this simply you could say
Amendment shall be permitted only in cases of: 1. deletion of one or more claims 2. correction of one or more errors.
Your example also raises another issue. It lists two conditions:
- deletion of claims 2. correction of error
There is some ambiguity as to whether these conditions are conjunctive or disjunctive as written. Does it mean
when there is both 1. deletion of one or more claims AND 2. correction of one or more errors
or does it mean
when there is either 1. deletion of one or more claims OR 2. correction of one or more errors, BUT NOT BOTH
or does it mean
when there is either 1. deletion of one or more claims OR 2. correction of one or more errors, OR BOTH
You need to clarify whether you mean
- AND 2.
- BUT NOT 2.
- AND/OR 2.
Best Answer
No it doesn't.
Let's analyse your example:
"Everyone" is the main bit of detail, and the main clause is everyone is required. If you remove including the staff from the sentence then you would obviously use "is". Since including the staff is somewhat separate to the main clause its purpose is little more than to add detail and so the sentence's syntax should be the same as if it were Everyone is required to wear shoes.
TL;DR - Use "is".