Well, let me start here. The rule against ending a sentence with a preposition is long dead and has been buried. You do not have to worry about it.
That said, the rule that says you shouldn't end a sentence with an unnecessary preposition is still in force.
Where is my scarf at?
That is still incorrect.
When it comes to who/whom there is a difference between AmE and BrE. The Brits are moving away from whom more quickly than the Yanks are. But the use of whom is falling on both sides of the pond.
a. 1 and 3 are grammatically correct. The first is a bit stilted. The second is common. The third is correct in formal English.
b. 4 and 6 are grammatically correct. The first is a bit stilted. The second is common. The third is correct in formal English.
C. All are incorrect. If you want a correct "whom" sentence that ends with a preposition, try "Whom do you report to?". "Who do you report to" would very common.