Situation: I have 1 unit damaged and two healing tactic cards in hand both able to heal that unit to full health.

I start an action chain and play a healing card.

(Opponent plays whatever action or passes, not really important.)

I have a card that triggers every time I play a tactic from my hand, so for the trigger I intend to play the other healing card (even if pointless heal-wise).

Am I allowed to play that second healing card? At first I would say no, as I have already healed my only damaged unit to full health meaning that there is no valid target for the heal.

But according to the action chain I have only played a healing card, which not yet resolved (it will resolve last as the last step of the resolve chain), so that the damage tokens are not yet removed from the to be healed unit. This meaning that even tho I healed the unit, I still have that unit with damage tokens on it, so I still have a valid target for my second heal card.

Second question:

Next, my opponent plays a tactic dealing enough non-combat damage to this damaged unit of mine to bring it down to 0 HP. Nothing more to add, the action chain resolves. My damaged unit is destroyed as the first resolution, and my two healing cards are futile without any valid target. Is that logic correct?

And just a last one I promise:

if there was another damaged unit, can the healing be redirected on that one if the original target is destroyed (or if I simply change my mind)? Or is it target locked when the effect was triggered? Or does it depend on the wording of the heal, whether it heals "target unit" or "any unit"? Or "any unit" only means I can choose freely, but once the choice is made, it is locked?

1) Again, as long as the card doesn't say "target damaged unit", an undamaged unit is a valid target for a healing effect. Your understanding of the chain is correct though. Even if the card required the unit to be damaged, you can play the second card because the effect of the first hasn't resolved yet.

2) Correct.

3) No. You choose targets when you trigger an effect. You cannot change them later, even if the ones you chose have become illegal. There is no Action in the game that says "any unit".