God gave the Apostles the authority to give the bishops the authority to impose a celibacy requirement upon future priests and bishops.
Where? Because as far as I can tell, Paul said that those who impose such things have "departed from the faith."
Now the Spirit expressly says that in latter times some will
depart from the faith,
giving heed to deceiving spirits and doctrines of demons,speaking lies in hypocrisy, having their own conscience seared with a hot iron,forbidding to marry, and commanding to abstain from foods which God created to be received with thanksgiving by those who believe and know the truth.For every creature of God is good, and
nothing is to be refused if it is received with thanksgiving;for it is sanctified by the word of God and prayer. - 1 Timothy 4:1-5
http://www.biblegateway.com/passage/?search=1Timothy4:1-5&version=NKJV
And no, Matthew 18:18 doesn't count, because that was only for the New Covenant God made explicitly with Israel.
Their duty is to preserve and transmit the teachings of the Apostles,
Such as 1 Timothy 4:1-5, no less...
and to administrate the Church's institutions, that were established by Christ and by the Apostles.
Which do not include forbidding to marry, aka, "departing from the faith."
What is the priesthood of Melchizedek? Did God not create that?
An unanswerable question. God doesn't tell us. Probably not, though.
Because they could afford to put into practice Paul's Apostolic counsel:
Which does not include "forbidding to marry."
It may be that soon, the Church will no longer be able to afford to impose this requirement upon future priests, that is up to the bishops to decide.
It wasn't up to them to begin with, nor to the authorities within the church.
And if they do change this discipline, they are only validly exercising their authority,
Wrong. They would be (unwittingly) correcting a mistake that had been made by those in authority so long ago.
which is what they're already doing in imposing the celibacy requirement.
Wrong. Such is, as Paul puts it, a "depart[ing] from the faith."