right, very complicated! do you already know what materials you will use? ist also a question if you can make a flexible mould or a hard one. if it could be a flexible one, a cast in one piece may be possible as well. my thought is, how to fix the tendons with the rest good enough not to rip of, especially when the mouth would open. another question would be what you want to have at the end: an animatronic head (I guess not right?), a head thad can do some posing, or just a sculpture. personally I am more fan of a "sculpture" and so I would prefer an unflexibel, fixed face with lips, tendons and so on cast in resin, because I don't really trust the duration of al kinds of rubber stuff :lol. but thats just my madness I guess.
on the other hand, if you could manage to glue the parts together, a separately molding would have some advantages. do you want to paint these parts at least or do you want to colorate the rubber itself a bit?