I completely agree with you here and appreciate your explanation.
My point is that I am not sure about "subject and-linking verb-sentences" like:
It is I/me who decides or
It is I/me who decide
where is the subject I should agree the verb with?
Do you think you could analyse this case, please?
The explanation here is the same as the one I gave before.
The sentences are cleft sentences containing relative clauses. So the antecedent is missing but we can "recover" it:
It is I/me, THE ONE who decides.
So you see, here your second sentence would be ungrammatical in English because the verb must agree with the singular of the missing antecedent, "the one".
I can't remember what your mother tongue is but what I have said here may not be the same in your language. In French, for instance, you don't have agreement with the missing antecedent but with the noun or pronoun that comes after "is":
C'est moi qui suis en retard, for instance.
It's me* who's late.
The form "suis" of the verb to be corresponds to the FIRST PERSON singular not to the THIRD PERSON singular as it does here in English.
This is because in English we make the verb agree with the missing antecedent: It's me, THE ONE who is late. But in French, as I said above, the verb agrees with the pronoun just before it (moi=me/I).
I hope this isn't too complicated but possibly the way things work in your mother tongue are preventing you from understanding what's going on in English for this type of structure. :-D:-D
(*or "I" if you are a stickler.)