Prayers for all involved!
Just a practical question maybe our priests can cover, is the retired Bishop now free to marry within the Church?
Yes.
If so, can any priest celebrate the Crowning - will there be Crowning? - or does the celebrating priest need special permission from the new Bishop?
Hard to say. For example, monks in the Little Schema are not allowed to marry even when allowed to return to the lay state. But these rules are no longer so hard and fast these days.
There is also a Canon which forbids marriage over the age of 70, so I imagine that the new bishop or the Synod of bishops may want to give permission to overlook it.
does defrocking in this case mean that the retired Bishop now longer has to pray the Hourly prayers, etc.?
It is not an obligation in the Eastern Church for monks or nuns or priests to "pray the Breviary" when alone or when travelling, uinless they chose to do so out of personal piety. It is viewed as a public office which should be served in a church.
does he still have the authority to act in emergencies (most laicized Catholic priests still have the authority to give Last Rites in an emergency)?
Not at all.