You are not allowed to resell or create a competitive product using stc.
For non-oss projects, all core members with push access should have apppropriate license. If then, kind of license does not matter.
No. If you register your business account to your personal account, you will get personal license which can be used for your private projects. In other words, if you are wokring on a compnay which buys the license for you, you don't have to buy it again.
The rule for OSS projects is not determined yet.