1. They press a button in the center to enlarge or shrink the ball, but I believe that a pokemon can hear what's going on outside of the ball and comes out at will. That's why none of the pokemon would come out of their pokeballs when asked to fight the diglett in the Kanto region. Also why Wobbuffet and Psyduck always came out at inappropriate times. I believe the return feature is voice activated, but there may be a small button that may not be visible on the cartoon.
2. They've been shown to return automatically, even when thrown from great heights like from hot air balloons. (See any team rocket attack)
3. Technically when a pokemon escapes capture it damages the pokeball. Think of the ball as a containment system. If the containment system is damaged, it can no longer contain the pokemon. Therefore, the pokemon is released and needs to be recaptured in a new ball. In most cases I would think an owned pokemon wouldn't care if it needed a new ball or not.
4. I have no idea. In a giant pile of pokeballs regained from a foiled Team Rocket theft, I would think that it would be easier to just have all of the pokemon come out and find their own trainers. Yet somehow every trainer seems to grab 6 pokeballs out at random and always get the right ones.
5. Why would that happen? That would probably be a pretty disorganized trainer. I guess they'd just have to use a different pokeball, but if they're just leaving $200 pokeballs sitting around and forgetting about them maybe they should consider a different hobby.
Edit:
Here's a Pokeball question I've always wondered about:
Pokemon are not immortal. This has been mentioned before. What happens if a Pokemon dies of old age while in a pokeball?
The only answer I've come up with is that they're contained as energy in the ball, so if the pokemon is no longer alive the energy fades from the pokeball. If that were to happen, the ball would deactivate or reset as a new pokeball to be used again. I wouldn't think that it would eject a dead corpse to mortify children... that would be just wrong.