There is a callback when using the Spawn Actor task that lets you set some properties on the spawned actor, so you can do it there. I would likely just have a “Owner” property on my projectiles, set it to the caster, and then when the overlap happens, use an Ability that just does the explosion vfx/sfx, and calls CalculateDamage which then passes in the owner and you can grab all the owner properties in the OnCalculateDamageForActor callback.
Heal would be the same thing. Just pass along the Caster either as owner or instigator, and then either use a custom event or Damage Task (which should work for negative damage…) and pull the stats there.