This a pretty simple one I think but I've been reading the forums and saw that you can't use a unit in combat more than once... You can't use a unit ability if the unit is spent. So if you spend the unit for blocking an attack, you can't use any other ability. Or if you spend a unit to get mana, you can't use any other ability.
I assume that it refers to attacking or Blocking effects, right?
Meaning I can use a ready unit to absorb damage in the assign damage phase, assuming that they have the right resistance and sufficient amount of armor... and then use them to attack for example...You don't "use a unit to absorb damage"; you just assign damage to it. It doesn't become spent. You can assign damage to a unit (only once per fight, even if it doesn't get wounded) and then, if it is still healed, use one ability. Or use one ability and then assign damage, even if it gets wounded.
Or I can use a Unit to block an attack (the unit becomes spent) and then use the same unit to absorb the damage of an attack in the same conditions as above (so they don't get wounded)If the attack is in the range phase, yes, you can. You can't do it in this order if you want to generate attack for the melee phase. In the range case, you can do it even if it gets wounded when assigning damage (Since you would have used it when it was healed).
Also I'm pretty sure you can't use a unit to attack/block, then ready it with another unit or a card and use it again to attack/block because that's considered to be healing and that would be considered using the same unit twice in the same combat, correct?Correct, effects that ready units are considered healing effects and cannot be used during combat. No exceptions.