Learn English – Plural of “copula”, does “copulas” or “copulae” sound more professional

Well, this is an example of why Google NGrams isn't a precise indicator. When we compare the two directly, aircrafts simply can't get off the ground:

Now, this result is also flawed since it is impossible to separate uses of aircraft (singular) and aircraft (plural). It is also impossible to factor out typos (aircrafts vs. aircraft's) and so on.

The point is, don't read too much into what an NGram shows (or at least take the graphs with a grain of salt), because a great deal of the time what you wind up with is this sort of thing:

(BTW, the huge spike in the use of aircraft in the early '40s is almost certainly due to the air war in Europe and the Pacific.)

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:

1. 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

• 1. AND 2.
• 1. BUT NOT 2.
• 1. AND/OR 2.