Por with passive voice

Question

Everywhere I look I see that you can use ser + past participle + por + agent to say who does something in the passive voice. My question is, can you only use por with ser in the passive voice? Is it wrong to use por with passive "se?" For instance, you can say:

Las galletas son horneadas por mi abuela.

BUT, can you also say:

Se hornean las galletas por mi abuela.

Is it wrong to do so or just uncommon?

Answer 1

You can't show the agent in an impersonal reflexive passive, because we don't care about the agent.

You can only show the agent when using the correct form of ser + past participle.

Answer 2

Se hornean las galletas por mi abuela.

I suppose some grammarians may consider this grammatically correct, but it is so uncommon that I have never anyone talk this way. It also makes no sense to say it this way because the purpose of passive voice is to remove the agent.