My guess is spring or summer of 2021, but I'm well aware that that puts me solidly on the optimistic side of timelines. My suspicion is that while Tesla hasn't announced the location yet, they're extremely active behind the scenes working on it already, including an extra emphasis on how to avoid / compensate for regulatory hassles.
The other important thing is that they likely have their orders in for all the known required long lead equipment.
Regarding the cell supply. Since Tesla is expanding their 2170 supply base (local sourcing for China production), they could potentially outsource the supplier agnostic Energy cells and switch the (qty) three or so lines at GF1 to controlled Automotive cells for 3/Y. Subject to supplier capacity, of course.