Both This mess was made by who(m)?
and your second sentence Who was this mess made by? are correct.
Your first sentence is not grammatical. You can say By whom was this mess made?
That's an interesting question. Normally, a switch to the passive voice allows the subject in the active voice to be dropped. In the case of your example, this may give rise to an unwanted inference.
If the original used courted instead of married, you could drop the subject and change the tense of courted to produce
- Lucy was courted four years ago.
However, doing the same with married to produce
- Lucy was married four years ago.
gives the impression that Lucy is no longer married.
This is the case with other verbs of the same type. That is, where using the past perfect looks like applying an adjective.
Marry verb
1 Join in marriage.
‘I was married in church’
‘my sister got married to a Welshman’
- ODO
Married adjective
1.1 (of a person) having a husband or wife.
‘a happily married man’
- ODO
You have the same phenomena with the word employ. Compare:
- IBM employed him last year.
- He was employed last year.
The way to get around it is to use the form in the second dictionary example above: got married:
- Lucy got married four years ago.
Best Answer
It's not possible to directly change the example sentence into the passive voice, because there is no transitive verb that's being used in the first place.
However, the most expedient way of making it passive is to simply insert a transitive verb:
If the agent of the production of honey (and its sweetness) is bees, then you'd have the following.