Can zihaar occur before consummation of the marriage? What is the ruling on one who divorced his wife by zihaar if he has intercourse with his wife before offering expiation?

Firstly: 

If a man states that his wife is as haraam to him as his
mother is or another mahram woman who is permanently forbidden to him, then
he has uttered a great evil and spoken falsehood, and he comes under the
rulings that result from zihaar. His wife remains haraam to him until he has
fasted for two consecutive months; if he is not able to do that because of a
legitimate shar‘i reason, then he must feed sixty poor persons. 

For zihaar to take place, it is not stipulated that he should
have consummated the marriage with his wife; it is sufficient for him merely
to have done the marriage contract with her for these rulings to come into
effect. 

Secondly: 

What appears to us to be the case is that what you said comes
under the heading of conditional zihaar. Because you decided not to cause
harm to that woman, then you come under the rulings on zihaar. So you have
to repent from saying that and your wife remains haraam to you until you
have offered the expiation for zihaar. 

The scholars of the Standing Committee were asked: 

There was an argument between my father and my oldest
brother, and my oldest brother said something, which was: “My wife is as
haraam to me as my mother.” That was in a moment of anger. My brother has
done the marriage contract with this wife of his but he has not consummated
the marriage with her and there has not been any wedding party until now. I
hope that you can advise me. 

They replied: 

If what is mentioned about your brother declaring his wife to
be haraam to himself is correct, and that happened after he did the marriage
contract with his wife, then he has committed a major sin and he has to
offer expiation for zihaar before he can touch his wife, whether he said
that before consummating the marriage with her or after. Expiation for
zihaar is freeing a believing slave. If that is not possible, then he must
fast for two consecutive months. If he is not able to do that, then he
should feed sixty poor persons, giving thirty saa‘s of wheat, half a saa‘
for each poor person. 

Shaykh Ibraaheem ibn Muhammad Aal ash-Shaykh; Shaykh ‘Abd
ar-Razzaaq ‘Afeefi; Shaykh ‘Abdullah ibn Ghadyaan; Shaykh ‘Abdullah ibn
Munayyi‘ 

End quote from Fataawa al-Lajnah ad-Da’imah, 20/277 

If you had consummated the marriage with your wife – as
appears to be the case – then you have to stop being intimate with her
immediately; it is not permissible for you to continue being intimate with
her before you have offered expiation for zihaar. 

The scholars of the Standing Committee said: 

With regard to what you have done of intercourse before
offering expiation as a result of ignorance before starting to fast, you do
not owe any penalty for that because you had the excuse of ignorance. But
now you must avoid having intercourse until you have completed the fasting.

 Shaykh ‘Abd al-‘Azeez Aal ash-Shaykh, Shaykh ‘Abdullah ibn
Ghadyaan. Shaykh Saalih al-Fawzaan, Shaykh Bakr Abu Zayd 

End quote from Fataawa al-Lajnah ad-Daa’imah, 20/322 

Thirdly: 

It is not permissible for a Muslim to follow the Shi‘i (Ithna
‘Ashari) madhhab with regard to basic issues or minor matters, or to be
happy if they are in agreement with Ahl as-Sunnah (concerning some issue),
just as it does not matter if they differ  from us. It is not permissible
for you to consult one who will advise you in accordance with their madhhab
or give you a fatwa based on their opinions. 

And Allah knows best.