Working at a dealership, if anyone asks anything, I say "It happens when it happens." Everything is just speculation.
As for the timing and model year, Corvettes seem to typically follow the actual year and don't do the "early" model-year thing like most models. Heck, the Equinox comes out with the new model year as early as May. The 2020 'Vettes may be built all the way up to Xmas, 2020. This won't make the 2020 Corvette a year old when you get it. Besides, it'll be so unique, it's not like it'll lose any value when you drive it off the lot. You can probably sell it for more than you paid for as soon as you get it, regardless of when you get it.