In the screen shot I posted you'll see I am using the Word Engine.
I intend retaining Word's dictionary (engine) as my primary spell checker source, the hunspell dictionaries will remain as a secondary option. So I'm currently focusing my attention on the Word Engine option.
In Word Options->Proofing>Custom dictionaries my Finance, Economics and Law custom dictionaries are unchecked, so I was surprised to see them in the Custom dictionary drop down. Are they being be used in the checking process - it doesn't appear so (nor should they be), can you confirm that?
If/when you provide an 'Add word to dictionary' I definitely don't want the ability to add a word to an 'unchecked' dictionary - e.g. to my Finance dictionary when I'm spell checking Swift's 'A Modest Proposal'
BR