In this sentence:
1- “It was he who came to the house.”
“It” , the dummy subject, represents “he”, the real subject.
The sentence can be rephrased:
2- “He was the one who came to the house.”
But in this sentence:
3- “He it was who came to the house.”
Why are the subject and the dummy-subject next to each other and still be grammatical?
I thought the reason we use a dummy subject is that it comes before the real subject.
In the sentence 2, can we replace “the one” with “him”?
Thank you!
piermo3- “He it was who came to the house.”
It is not grammatical English.
piermoIn the sentence 2, can we replace “the one” with “him”?
OK 2- “He was the one who came to the house.”
Wrong. 2- “He was him who came to the house.”
piermoIn this sentence:
1- “It was he who came to the house.”
“It” , the dummy subject, represents “he”, the real subject.
Hmm. This is a cleft sentence. The initial 'it' is a dummy 'it', but it doesn't represent (refer to) anything. Cleft sentences have the pattern
It is/was {highlighted element} that/who/which {residue}.
Compare, noting how three different elements may be chosen for highlighting in this example sentence:
John and Mary brought the table into the living room.
(sentence to be rearranged into cleft structures)
Possible clefts from the given sentence:
It was John and Mary who brought the table into the living room.
It was the table that John and Mary brought into the living room.
It was into the living room that John and Mary brought the table.
piermoThe sentence can be rephrased:
2- “He was the one who came to the house.”
Yes. That rephrasing is the usual way to say it.
piermoIn the sentence 2, can we replace “the one” with “him”?
No.
piermoBut in this sentence:
3- “He it was who came to the house.”
Why are the subject and the dummy-subject next to each other and still be grammatical?
What a curious sentence! I can't be sure I would even call it grammatical. It certainly doesn't follow the pattern for cleft sentences. If I were you, I'd forget about this sentence. You won't need this pattern more than once in 500 years. 
There must have been some very unusual purpose the writer had in mind for using that inversion — maybe to focus attention on "he", in the same way as adding "indeed" might do — or maybe in poetry, where inversions are more common.
— Was it he who came to the house?
1) — Indeed, it was he who came to the house.
2) — Yes, indeed. He it was who came to the house. (?)
piermoI thought the reason we use a dummy subject is that it comes before the real subject.
That's the usual plan. Yes. But recall that dummy 'it' doesn't refer to a subject. It just replaces a subject. It's a place-holder more than anything else. You could definitely not write He was who came to the house.
CJ
Good. That shows you have good judgment.
CJ