I think the biggest unknown (almost) is what pier comes next. If pier two is built next then presumably most if not all the legacy contact gates on T2 will remain available. This would make sense, since pier two can open with it’s available space and then the legacy gates close for the final pier construction.That depends on 2 things. Firstly, and very obviously the tie up with Virgin but secondly, and more importantly, the provision of a domestic passenger facility within T2/T2X. If the domestic facility stays in T1 then, if BE were to park remotely at T2X, passengers would need to be bussed and nothing is gained - if anything the passenger experience will be worse.
Also think about the lack of apron space around T2X? Where would you put the flybe aircraft. So, we are back to the date of around 2022.
By next summer there will possibly only be a very few stands in use for boarding dir3ctly from the old T2 frontage. I suspect that the west apron may have gone but we will gain the new Lima remote stands.
I'm hopeful that @marni1971 will join in on this discussion as I believe he may have some useful input.
Also worth noting that the original (approved) T2X plans had a domestic element.