The Daltaí Boards: Which, who, whom, that

Just want to clarify what word you should use for "which, who, etc." and when.

Firstly, please correct me if I'm wrong with any of the following.

A) I went to the building which he mentioned.
A) Chuaigh mé chuig an aras a luaigh sé.

B) That is the man I got the letter from. That is the man from whom I got the letter.
B) Is é sin an fear a bhfuair mé an litir uaidh. Is é sin an fear óna bhfuair mé an litir.

C) That's the door I closed.
C) Sin an doras a dhún mé.

D) That's the girl whose book I bought.
D) Sin an cailín ar cheannaigh mé a leabhar.

E) That's the box I put the letters in. That's the box into which I put the letters.
E) Sin an bosca ar chuir mé na litreacha ann. Sin an bosca inar chuir mé na litreacha.

Are all of the above okay?

I've seen people use "go" and "gur" instead of "a"... is that acceptable? eg:

-Sin an buachaill go dtaitníonn léi.
-That's the boy she likes.

Is the above okay? Or can it only be used where you would have had "a + urú" or "ar", eg:

-Sin an buachaill go taitníonn peil leis.
-That's the boy that likes football.

How do you work with the copula? Please correct any of the following if they're wrong:

A) That's the boy who is a doctor.
A) Sin a buachaill is doctúir é.

B) That's the book I like.
B) Sin an leabhar is maith liom.

C) That's the boy whose father is a doctor.
C) Sin an buachaill ar doctúir é a athair.

D) That's the book whose cover I like.
D) Sin an leabhar ar maith liom a chlúdach.

E) That's the boy who likes jam.
E) Sin an buachaill ar maith leis subh.

F) That's the boy whom the girl likes.
F) Sin an buachaill ar maith leis an gcailín.

Are the above all OK? What are the guidelines for using "gur"? Are they the same as for using "go" instead of "a" with normal verbs?