Kitchens – place ventilation ducting on top of joists if there is space

This entirely depends on the refrigerator and where the coils and fans are. Older refrigerators were cooled with fins on the back, and depended on convection (heat rising). The heat of the coils would draw air through the lower grille, up the back, and over the top. A modern unit might do all it's cooling with a fan pushing heat out at the front bottom plate.

Samsung's current models require 1" on top, half inch each side. See : http://www.samsung.com/us/support/faq/FAQ00021650/21818/Y/RB197ABPN/XAA

Keep in mind your cabinet is now built to fit just one size refrigerator: just hope that size is available in fifteen years when your current model is no longer repairable due to planned obsolescence. Front venting units are also somewhat less efficient, all other factors held constant.

It's common to have the cabinet above the fridge a bit less deep than the others, to allow for airflow.

This question really begs to have the analysis done by a structural engineer. That being said I can venture to suggest two ideas that may be suitable to your situation.

A) Probably the best way to provide adequate support for the drywall on the ceiling would be to screw flat 2x4's at right angles to the existing rafters onto the bottom of the lower rafter stringer. Place these on 16" centers so that you get the right spacing for the new ceiling materials.

B) If you try to add in 2x4 stringers in between the existing rafters you would definately not want to have those stringers have an unsupported span that was less than the segments on the lower side of the existing rafters. This likely means that a single cross header near the center is not going to be suitable. The rafter construction often has the support design of the rafter webbing in the shape of a W that divides the lower stringer into three sections. Your cross headers should be at points that the rafter webbing intersects the lower stringer of the rafters. Personally this method is so much more work than method A above I would try to avoid it.

You should check with your local building code administration to see if you can even use plywood to cover a garage ceiling of your style. If your garage is attached to your house there are often stringent regulations that specify the use of drywall at least 5/8" thick.