SPOILER (click to view)

he can only use it for a short amount of time because he hasnt completely mastered the skill yet. The arrancars (i think thats what they were called) teach him how to use it because basically they are the same as ichigo in which they can turn into a hollow/human hybrid.

sonryu

In the manga now (which I guess you don't read), even though it's for a short period of time, Ichigo CAN control his Hollow and use Bankai (not animated yet). I think he may be more powerful when the Hollow completely takes over though (not animated yet either). It's hard to tell how strong he is though because in the manga, every time Ichigo gets stronger, there's always a much stronger opponent for him to fight.

SPOILER (click to view)

no...IIRC, Hirako said to Ichigo before they started his hollow suppression training, that an inner hollow cannot be controlled. it's not something you can train your body or mind to do. you can only control their powers, not them. Ichigo can only hold it for a short time...i'm guessing it hasn't become a natural form for him to take on at this point. also mentioned in the newer chapters that the form exhausts his reiatsu really fast.