1. In general, 1 license key can be activated for 1 project twice (production and development). If in your case, 1 project consists of 2 domains, so you can activate them in such a way if another domain is related to another project, you need to buy an additional license.
Please clarify about your second domain, is that a part of the same project?
2. Yes, you can activate the theme when it is ready.