Why not reword to avoid confusion?
...within the callback, the length of
charshould always be 1.
and, as you suggested,
...
charinstances are defined in multiple scopes...
Here are some style guide references to support my answer.
Yahoo style manual recommends re-wording, as does reference.com, and The manual of scientific style: a guide for authors, editors, and researchers By Harold Rabinowitz section 3.6.7.ii-iii
On the other hand, the Chicago Manual of Style says
The plural of a word or phrase in quotation marks is now formed without anapostrophe—that is, with the addition of s or es within the quotation marks. Note the example: “To be continueds”
Although it does not address possessives of items in quotes.
