There's still nowhere to play until Oct 2020, though. October 2020, meaning that if they're planning on a 2020-2021 season debut, they can't have more than two weeks of construction delays prior to interrupting the preseason schedule since there's nowhere to play home games. 20-21 debut means Offseason 2019 announcement if they grant it a year ahead of time like Las Vegas. For political reasons, I don't imagine they're going to let Seattle scouts go all over the world for two years, especially with what the Knights are doing with only one.
I think it's more likely that Key renovations are delayed than they go perfectly since that's the way construction (and especially Seattle construction) goes. I think they wait past this summer just to make sure they're not announcing/granting a team three years prior to them hitting the ice if there's a couple of month delay.
The NHL will be in Key Arena first, but even if they're not, I don't think it matters. Hopefully the ticket drive goes well and they'll be fine regardless.
When MLS announced MLS is coming to Seattle it was announced fall of 2007 they began playing Spring of 2009. So its not far of what other leagues have done MLS is usually 2 years between announcement and when they begin playing.
The thing is this is expansion not relocation, the league can set when the team starts playing which will be 20-21 season,
Don't compare Key arena reconstruction to any other construction in the Seattle region when the fact the shell of the arena is staying there, that saves time vs a 100% tear down and rebuild (obviously that isn't possible due to landmark) but it shouldn't take that long to dig down another 15 feet get the foundation poured in and what not.
There is more to it then just the team side of things, there is the business side, getting upper management in place, who is going to run the business side of the franchise, getting the sponsors in place, tv deal.
The website suggests a 2018 team announcement, with season ticket prices announced early 2019.
Regarding pre-season games, nothing stops them from playing them in Showare center or the Everett's arena. They can also have the first few weeks of the season on the road if it comes to that.