Verb agreement in "[adjective] and [adjective] [noun]" constructions

Solution 1:

When you say

where the participle and the adjectival form differ

you've chosen a rhetorical device called prozeugma, which means that preceding phrases (here, "the participle" and "the adjectival") govern a single following word (here, "form"). What this means is

where the participle form and the adjectival form differ

As you've noted, the choice of the number of form belongs to the author, and you've chosen the singular. You could have chosen not to use prozeugma, and instead written the plural forms, making "participle and adjectival" a single compound modifier.

But no matter which choice you make, you still have a compound subject, two things that differ from each other, which makes two differences, one from each point of view. That requires a plural verb, namely differ. It's true that the singular noun form might be slightly jarring next to the plural verb differ, but grammatically it's the plural subject that matters.

It is possible to use the singular differs, but you'll have to rephrase so that the verb is governed by a singular subject:

where the participle form differs from the adjectival form

Of course it's still true the other way around, but grammatically, you speak to only the one difference, so the verb must be singular.