Can a character be healed back to healthy once they have been wounded?

I don't think so. You can be healed, at least I've been playing that way, back to full wounded health, but you can never turn the card back over.

There are basically 2 standard states your character can be in.

(In some campaign missions there is a 3rd)

1) Healthy

2) Wouinded

You are healthy until you have taken enough damage to equal or exceed your maximum health.

When you suffer damage equal to or more than your health, you flip and become wounded.

All damage is cleared from your card, and you continue to play in this new wounded state.

So far I don't know of anything that would flip you back from wounded, although I believe the Droid Hero can prevent the wounding from taking place?

No. Once defeated, the hero is wounded. There is no mechanism other than ending a mission to flip the hero back to the healthy side.