Q1: Is this a reasonable/acceptable plan? Yes & no...more below.
Meets code? You'll need to run all of this by your local/city inspectors/building code enforcement folks to get the answer.
Energy efficient? Roofing-wise, I believe it is. Several inches of foam will be quite efficient--but that may be undone by what your other choices are ...
Yes, you can add insulation without removing the plywood.
There are several issues you’ll need to address: 1) ventilation, 2) vapor barrier, 3) recessed ceiling lights
1) Ventilation in the attic is critical. The code calls for “cross ventilation”. That means soffit vents, gable end vents, or both. (Ridge vents can supplement soffit vents or gable end ...
I'm assuming that the floor of the attic is insulated, and the roof is not insulated at all. Do I have the right picture?
A: Put insulation on top of the plywood. This is straight forward, and can be done with soffet baffles at the edges, and half a day's work with an insulation blower. You can choose between cellulose and fiberglass. The former ...