It is likely an email will be sent around detailing all of this closer to release - the game has recently been greenlit, so it lis likely that Stonehearth will be on Steam at some point - but they’ve stated they would want a way for people who don’t use steam to access the game. As such it sounds as if there will be a standalone launcher for the game.
Yes. They’ve said the beta release will have the two core elements of crafting and building in place. As such it is likely that we will have updates from beta up until release and beyond adding new features and improving the game.
Mac and Linux was one of the stretch goals. It will be on Mac.