Question Words
When was the reservation made? (correct).
When was made the reservation? (incorrect).

"When the reservation was made?" (Can anybody tell me if this question is well formed?)

Question

Question Words
When was the reservation made? (correct).
When was made the reservation? (incorrect).

"When the reservation was made?" (Can anybody tell me if this question is well formed?)

Answer

Sorry, your question is not correct.  Your first sentence is the only correct option for the question.

You can say this: "When the reservation was made, I was asked if I want to pay for breakfast too."

Here "when" is not used in a question and so the word order is the same as for other statements. .

Answer

It isn't. In English we do not have this kind of flexibility in question forms. Only the first question is correct.

I can see from your question that you haven't been taught the basic formula for interrogative forms. This is the standard word order:

1.  Question word:  When
2.   Auxiliary verb:   was
3.   Subject:  the reservation
4.   Main verb: made?

This is a very simple formula, and it works for all types of questions. You need to learn this 1,2,3, 4  formula and use it whenever you make a question.