Cases are fully compatible as the external dimensions of both computers are identical.
It looks like keyboard covers for 2018 are compatible with 2019. I ordered a cover yesterday and looked into this a bit. From what I could tell from various sites, it depends on the type and design of the cover. Some cover sites listed compatibility for one product as 2016-2019; others broke it out into 2016, 2017, and 2018-19. In either scenario, 2018 looks to be compatible with 2019.