Remember, that is fairly unlikely. VA Dems need to win the governorship in both 2017 and 2021 or get a 21 seat majority in the state senate in 2019 in order to be assured of a say in the 2022 congressional redistricting. For state legislative redistricting, they could force a neutral map for one cycle in 2021, but the the GOP has a trifecta after 2021, they will redraw in the next legislative session.
I agree with a number of your points, but I'd like to add:
1. I'd say the odds of Democrats holding the Gov/LtGov in 2017 as pretty good if Trump is still deeply unpopular by next year. 2021 is anyone's guess.
2. Democrats have fairly good odds of winning 2 - 3 seats
(SD-13, SD-17 & SD-20) in 2019. Again, if Democrats can sufficiently nationalize these races around an unpopular Trump, chances might be even better.
3. State Senate elections are held every 4 years, so if they win the majority back in 2019, they will hold it until 2024. Any map changes would have to come after then, or longer if Democrats again hold the gov's office.
4. You're right that the GOP will probably try to change the maps, as they made several attempts under McAuliffe, but the state constitution is pretty unambiguous that districts are to be redrawn every 10 years. Democrats could file a lawsuit in state court, and given how patently absurd the recently 4-3 ruling overturning McAuliffe's voting rights restoration order was
(again the state constitution was extremely clear that he had broad discretion on this power), Democrats might benefit from using a possible State Senate majority to deny "reelection" of McClanahan to the VASC. She supported a ruling in favor of overturning McAuliffe's order despite there being a possible lack of standing and absolutely no constitutional basis. I'd venture a bet that she again ignores the constitution in such a mid-year redistricting lawsuit.
* I'd also like to note that dozen(s) of Virginia legislative
(SS & HD) districts are pending review by the US Supreme Court in a gerrymandering lawsuit, and it's possible that the map could change even before the end of the decade.
