If blockers have already been declared when Stoneshock Giant's last ability resolves, those blocks won't change or become undone.

Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.

Monstrous isn't an ability that a creature has. It's just something true about that creature. If the creature stops being a creature or loses its abilities, it will continue to be monstrous.

Your opponents can't assign any creature without flying to block that turn, even if that creature had flying or wasn't on the battlefield under one of your opponent's control when Stoneshock Giant's ability resolved.

An ability that triggers when a creature becomes monstrous won't trigger if that creature isn't on the battlefield when its monstrosity ability resolves.