If it helps you sleep at night, sure. Personally, I don't think it makes a bit of difference if you have a half quart or whatever of old fluid left in there unless the fluid you drain is horrible miscolored and you pulled a lot of metal off the drain plug.

 

Do you mean pour back in the fluid that you just drained out? No, there wouldn't be any point to that...

You need the new fluid to mix with the old stuff that's stuck in the torque converter. That's why you drive around between drain/fill cycles.

If the fluid that drains out is pretty clean, then just don't bother draining again. If it's really dirty, maybe drain & fill more than 3 times until it comes out clean.

Imagine a bottle that can only be drained 1/2 way. How many times will you drain & fill it before it's clean enough to drink??