Matthew 18 and Deuteronomy 19: At the mouth of 2 or 3 witnesses

15 Moreover, if your brother commits a sin, go lay bare his fault between you and him alone. If he listens to you, you have gained your brother.
16 But if he does not listen, take along with you one or 2 more, in order that at the mouth of 2 or 3 witnesses every matter may be established.
17 If he does not listen to them, speak to the congregation. If he does not listen even to the congregation, let him be to you just as a man of the nations and as a tax collector.
18 Truly I say to you men, Whatever things you may bind [prohibit by legislation] on earth will be things bound in heaven, and whatever things you may loose on earth [permit by legislation] will be things loosed in heaven [true after was appointed in John 20:22-23].
19 Again I truly say to you, If 2 of you on earth agree concerning anything that they should request [to be bound or loosed in heaven. I.e. a judicial matter in the congregation including the setting of church doctrine], it will take place for them due to my Father in heaven [So the head of a judicial committee cannot proceed without at least one other member agreeing with him and possibly even two other members. This is not democracy. But then neither is it a dictatorship].
20 For where there are 2 or 3 having been lead together into my name, there I am in their midst [one cannot be in the midst of one person, one can be in the midst of two or three or more] [This is all to do with the meaning of midst. is there when one person is lead into his name, but he is not in the middle of that person. Whereas with 2 or 3 he can be in their middle, for they are connected by their love for him. He is the common factor. is the pivot that balances two brothers. One brother will have prejudice of one type. The other brother will have different prejudice. If they both co-operate in love and righteousness through , who has perfect balance which they aspire to, they will be helped through a 3-way interaction to overcome their prejudices and reflect more accurately the character of the Christ. The whole becomes greater than the sum of the parts through the holy spirit. moderates the character of both brothers to reduce their prejudices. He cannot do that with one brother.] (Matthew 18).

The idea that the kingdom of God or is actually inside each one of us is unhelpful in understanding Matthew 18. The extra agreement requirement, the non dictatorship clause, is obviously critical to understanding God's M.O. And no church has ever implemented it properly in the past. So all mankind and all Christians have failed to grasp God's law. And now it is too late for this system. So we stand condemned as negligent failures. However God is training us now for the next system. It is plainly his will and it is a good teaching method to get us to work out the intricacies of his law for ourselves. So those who are interested enough to do this will be the kings and the judges for 1,000 years in the next system.

Now let us consider the headship of the man in family unit. The law above does not directly apply since deciding which TV program to watch is not binding in the heavens. But if there are two adult family members over whom the head has authority, the advice would be to take at least one with him in any decision he makes. The man is not advised to have to listen merely to his wife. But if his wife and his adult daughter both disagree and no other adult family member agrees then he has a problem. 

15 No single witness should rise up against a man respecting any error or any sin, in the case of any sin that he may commit. At the mouth of 2 witnesses or at the mouth of 3 witnesses the matter should stand good [in some cases 2 witnesses are sufficient, in others due to the gravity or perhaps the subjectivity, 3 are needed] (Deuteronomy 19).

16 In case a witness scheming violence should rise up against a man to bring a charge of revolt against him,
17 the 2 men who have the dispute must also stand before , before the priests and the judges who will be acting in those days.
18 And the judges must search thoroughly, and if the witness is a false witness and has brought a false charge against his brother,
19 you must also do to him just as he had schemed to do to his brother, and you must clear away what is bad from your midst.
20 So those who remain will hear and be afraid, and they will never again do anything bad like this in your midst.
21 And your eye should not feel sorry: soul will be for soul, eye for eye, tooth for tooth, hand for hand, foot for foot [because failing to do this will cost the community a lot more than one soul or one eye or one tooth or one hand or one foot] (Deuteronomy 19).

2 Witnesses are needed for a warning from the congregation
3 Witnesses (or 2 Witnesses one of which is a confession from the accused) are needed for a disfellowshipping (a capital offence)

25 If, however, it is in the field that the man found the girl who was engaged, and the man grabbed hold of her and lay down with her, the man who lay down with her must also die by himself,
26 and to the girl you must do nothing. The girl has no sin deserving of death, because just as when a man rises up against his fellowman and indeed murders him, even a soul, so it is with this case.
27 For it was in the field that he found her. The girl who was engaged screamed, but there was no one to rescue her. (Deuteronomy 22 )

Only 2 witnesses. Yet law says if he did it he must die by himself.
So 2 witnesses are sufficient only if you get an admission of guilt. Otherwise it must be 3 witnesses.
We must become experts at trapping abusers. But we can only give a warning if there is no 3rd witness and no confession.