Entities should work just fine, I too would love to hear about these error messages.
Note: calibre's editor has an interesting feature -- type in a valid entity, and it will be automatically turned into the unicode codepoint when you type the closing ";" as well as when Beautify does a pass. Also, Smarten Punctuation is builtin.
Very useful for seeing what's what at a glance, to turn the entities into the actual characters in question.