I don't know. I am not a native speaker, and am not an English major, but the answer is "yes" to both of them.
Am I understanding you correctly that you are saying people can't make out active and passive between "I wrote a letter", "A letter was written by me"?
"Who" and "whom" is a bit tricky, but I follow the rule about "who -> he"(who broke the vase? he did), "whom -> him"(whom do you trust? him). There might be edge cases I am unaware of.