Demo version with one mission will be released in 2021. Full version release date will be not sooner than 2022.
No. There will be only single-player campaign and additional missions.
There is no Garrett in this setting, you will play as another thief character.
Thievery: Beneath the Veil of Night is set in another universe than Thief series. Thus there are no Keepers, no Hammerites, no Pagans and city of Kartmar is different from the City. However there are factions reminiscent of them through different and general atmosphere of Kartmar is close to the one of the City. The setting is clearly medieval, not Victorian as in Thi4f of 2014, or too close to Victorian as in Thief II: The Metal Age, but there is some amount of steampunk -- similar to the level of Thief: The Dark Project and Thief III: Deadly Shadows.