Hey Falott

Glad to see the questions coming out here. For the inner mouth or mouth cavity which ever you prefer, I personally model it as an extension of the lips so its all part of the head in the end. Teeth and gums I would model separately. They are easier to manage this way and you will only need to key the lower set of teeth with your facial expressions on the mouth, be it in RX or RZ axis and possibly the odd TX or TZ too.

Its not difficult to do, if you get stuck once you are there give us a shout

_J