While "I" is the subject of the main clause, "who/whom" is part of the relative clause refers to the object of the preposition "for". The subject of that clause is "the cafe".

It's easier to see if we split the two clauses in your sentence into two separate sentences. It now looks something like this:

I am the one. The cafe was kept open for me.

"The cafe" is the subject of the clause with "for me". The word "me" is the object of "for". To connect the two clauses, we first replace "me" with an appropriate relative pronoun:

I am the one. The cafe was kept open for whom.

"Whom" is appropriate here because it's the object of the preposition "for", and "for who" is bad grammar.

Next, we move "for" and its relative pronoun object to the front of the clause:

I am the one for whom the cafe was kept open.

Although "I" and "whom" happen to mean the same person, grammatically they are two different nouns from different clauses.

While "I" is the subject of the main clause, "who/whom" is part of the relative clause, and refers to the object of the preposition "for". The subject of that clause is "the cafe", not "I".

It's easier to see if we split the two clauses in your sentence into two separate sentences. It now looks something like this:

I am the one. The cafe was kept open for me.

"The cafe" is the subject of the clause with "for me". The word "me" is the object of "for". To connect the two clauses, we first replace "me" with an appropriate relative pronoun:

I am the one. The cafe was kept open for whom.

"Whom" is appropriate here because it's the object of the preposition "for", and "for who" is bad grammar.

Next, we move "for" and its relative pronoun object to the front of the clause:

I am the one for whom the cafe was kept open.

Although "I" and "whom" happen to mean the same person, grammatically they are two different nouns from different clauses.