All grading should be done on the gamut the display can actually achieve accurately.
At the present all Rec2020 deliverables are graded P3, and converted to Rec2020 later.
Theoretically you could use Rec709, if that is all the display you have can achieve, but I doubt any broadcaster would accept that.
The second part of your question actually makes no sense.
Can you better define the question?
Mob Boss at Light Illusion