a) The commercial license lets you legally use Obsidian in a workplace setting. Details in the link below. It does not include Obsidian Sync or Publish.
b) Obsidian “vaults” are just a folder on disk. For Desktop ↔ desktop sync, you have lots of options. Desktop ↔ mobile less so, but there are a handful.
c) These are the Sync plans and discounts as of now: