WHEN A FRIENDLY UNIT WOULD BE DESTROYED

At what point are you able to trigger abilities/tactics cards with the trigger “When a Friendly Unit would be destroyed”?

Models are taken off at the same time. You are able to trigger the effect immediately when the unit suffers enough wounds to be removed from play. For example, if you have 4 wounds left on an infantry unit and it suffers 4 wounds due to failed defense saves, you resolve the effect of your “When a Friendly Unit would be destroyed” trigger before removing the 4 models.

Follow up: If during the interrupt step of “would be destroyed” effects resolving, the unit that would be destroyed suffers wounds, is it able to be destroyed?

Since the timing is before models are removed, suffering wounds during this time would trigger removal of models, and if enough models are removed to destroy the unit then yes.

If you get to perform an attack during "would be destroyed" trigger, can you stay alive if you heal enough wounds?

No. If you trigger "would be destroyed" or "is destroyed" effect, you cannot stay alive by any other way then the one provided by effect itself (such as, It Shall Not End allowing you to stay alive if you destroy enemy unit)