X coverage is how thoroughly you have examined the product with respect to some model of X.
Since we use risks to think about our test strategy, risk coverage is part of our thinking. Risk coverage is “how thoroughly you have examined the product with respect to some model of Risk”
in other words: which risks have we tested? I do not see many people talk about this which is unfortunate. Test missions should be derived (amongst other things) from risks. If you start with risks, I think it is useful to report (which can be verbal, stop thinking in documents 
) on those risks and what you did to examine them and which risks you think are still there.
You can also think of requirements coverage: “how thoroughly you have examined the product with respect to some model of requirements”. In other words: which requirements have we tested? I like to talk about product coverage: “how thoroughly you have examined the product with respect to some model of the product”. In other words: which elements of the product did we test? To be able to talk meaningfully about coverage, we need to model the product. The Heuristic Test Strategy Model mentioned in question 1 can help you with this.
Test coverage is the sum of all coverage mentioned above.