Kelli, as cdttmm said, they don't have to be ground. If you chose to grind them, you could improvise, especially since neither mustard nor coriander seeds come with the kinds of husks you see on cardamom or nutmeg. Even a rolling pin or a heavy glass jar bottom would do the trick, though it would be more work than a grinder/mortar & pestle.

As for the coconut milk, it still can go in a lot of dishes. Asian and Indian dishes tend to use non-sweetened coconut milk. But the sweetened kind probably could be cut with water or stock to make it usable. Or just save it for desserts and sweeter dishes.