Yes, of course it is allowed. But maybe it isn't smart. If you give the URL of the free blog site I can tell you if it is smart to use it or not.
Absolutely... just make sure you follow the rules. Otherwise, your ads be blocked or slapped with high CPC.
It's allowed, but not smart. You want to own all your landing pages. Instead of putting the landing page there, put a blog linking to your real landing page that you own. That would be far more beneficial and less risky in the long run.