If Democrats fail to win the House in 2018...

[Skill and Chance]

No later than 2022, else I don't think its gonna happen.

Well, they could narrowly lose next year and then narrowly beat Trump in 2020 while falling short again.  If the new Dem president gets reelected and it's not a huge landslide, I would expect that Democrats don't pick up the House until the 2030 midterms.

[Skill and Chance]

They could fall short again in 2020 despite winning the White House, then could be going against strong headwins in 2022 and 2026.  I suppose they could win in 2024 to coincide with a Democratic President's reelection.

Maybe, but how Gill v. Whitford turns out will factor in quite a bit here. Should SCOTUS side with the plaintiffs and a test be developed, there will be a huge wave of partisan gerrymandering lawsuits starting next year. A lot of Republican-drawn maps will be softened up for 2020 if that is the case.

However putting that aside, coming up short in both 2018 and 2020 would require some exceptionally weak performances. We're talking about 24 seats here, and if Democrats couldn't win that many with both a favorable midterm and a favorable presidential election happening consecutively, well, I wouldn't even know what to say.

I think you need to be prepared for a big let-down vis-à-vis Gill v. Whiford. Don't underestimate the party loyalty of the five Republican Justices -- Roberts, Kennedy, Thomas, Alito, and Gorsuch. Don't forget what happened in Dec. 2000: Bush v. Gore.
Just substitute Roberts for Rehnquist, Alito for O'Connor, and Gorsuch for Scalia, and it's the same bunch who stole from Al Gore his right to find out whether he actually won or lost.

Bush v. Gore would never have happened if the networks had not called Florida an hour early and supressed voting from the panhandle.

I do believe this is true.  It's plausible that several thousand people in the central time zone didn't bother to rush out and get in line at the polls because of that TV announcement.  Those people would have presumably broken at least 2:1 R.