Regarding the Tai-Chi issue, my personal opinion is that it is definitely a MA. You can see in Aikido that the most basic and simple techniques, the ones with the least martial intent, such as Kokyu Ho for instance, are the most benefitial ones. After practicing "meaningless" form diligently for ages, one becomes naturally a "master". Tai-Chi takes this concept at an even higher level since exercices are not paired. That's why "mastering" Tai-Chi takes a life time, but I have no doubt that Tai-Chi masters are highly efficient martial artists. Aikido and Tai-Chi are known to be internal MA, which efficiency looks doubtful in the beginning (the first 20 years or so), but in the long run prove to be superior to other more basic arts.
It is curious to notice that techniques which are directly meant for self-defence, and practitioners who put emphazis in their training on such techniques, instead of the more abstract but purposeful techniques, do not take go very far in terms of mind-body control, and technical over physical, or Ki ...etc.
Sorry for this very incoherent post. I hope you'll understand anything