Clarification about passive

Doing some exercises about the passive transformation (rewrite sentences from active to passive), I came up against some problems.

The problem sentences are:

• Martin always wears casual clothes

I wrote:
Casual clothes are always worn by Martin.

But it's wrong. The author's book says: Not possible to rewrite the sentence. But I can't understand why.. If the sentence were: "Casual clothes are worn always by Martin", (maybe) I would change the meaning of the sentence.. It would seem like "Martin is the unique person wearing casual.."

• Most of the guests had left the hotel by midday.

My sentence: The hotel had been left by most of the guest by midday.

Author's opinion: Not possible to rewrite the sentence.
Again, I can't understand why...

• Some parents read to their children every night.

My answer: Not possible to rewrite the sentence, because no object of 'read' is given in the sentence, so it's intransitive.
Author's answer: Some children are read to by their parents.

I reviewed the examples in the book and

• They sent me a letter -> A letter was sent to me.
  But it's not my case, because here the verb has an obj.




• People shouted at the Prime Minister -> The Prime Minister was shouted at. Here the verb is intransitive (like in my case).. Do I have to conclude that if the verb is intransitive and it is followed by a preposition, I can always rewrite the sentence from active to passive?

(I'm studying on "First Certificate Language Practice, Vince - Heinemann")

(Additional question: Is 'casual clothes' singular or plural?)