Having everything working as expected/required is probably simply a precondition to a potential purchase — so nothing wrong with that.
Mp3tag's Tag Sources can only query what's retrieved with the API request to a particular release. The original release date on Discogs seems to be tied to the master release, which could — in theory — also be used to import the data (but is currently not, and would not include the data of the actual release then).
The square brackets are special characters in Mp3tag's format string language, so you'd have to escape those as in
music/%artist%/'['%year%']' %album%
Extra discs depend on how you store those in your tags, you could, e.g., use
music/%artist%/'['%year%']' %album%[/%discnumber%]