The key is the verb "to make" in this case. It's followed by the bare infinitive - to make [to] agree to do
If instead of "make" you use "force," which is followed by the
complete infinitive, you can better see how the form works.
Someone tries to force someone to agree to do something. There are three "complete" infinitives in this sentence. The only simple present verb is "tries," which follows your rule. Infinitives don't change from singular to plural.
Furthermore, the second "someone" is the direct object of "force," rather than a subject, and therefore is not required to agree with the verb.
The same thing is true of "someone" in the definition you have quoted from the dictionary.
Another example which follows your rule would be:
Unless someone agrees to pay for this meal, we'll all have to wash dishes. Here, "someone" is the subject of the clause, and the verb is singular, to agree.
I know I didn't approach you question from the right end. If your dictionary quote were something like, "in the movie, someone agrees to do something terrible," "someone" would be the subject of the sentence and would follow your rule. But in your example, it is
not the subject.
- A.
Edit. I hope you can follow this mess. I should have scrapped it and started over, but I have to go to work! Sorry.