Possible Infinite Loop?

So let's say a card is played that does huge damage to everything (like Powered Shockwave when Unity has a lot of bots out) against Apostate with Relic Spirit on the table and only Runes of Malediction in the trash.

The attack hits the Relic Spirit, destroys it, and Runes of Malediction is brought out of the trash. The attack is still going on, so it then clobbers Runes of Malediction, destroying it. When it dies, it pulls Demons out of the trash, which would revive the Relic Spirit.

Does the attack continue by hitting the Relic Spirit, starting the whole thing over again?

Yes and no. It would hit the Relic Spirit again, but the loop wouldn't continue.

That loop was covered in the fireside chat under Infernal Relics section

https://greaterthangames.com/forum/topic/clarifications-and-decisions-from-handelabra-christophers-fireside-chats-6773

The wording was changed on Runes of Malediction and Relic Spirit.   Relic Spirit says that it can only bring back relics the first time

Ah, gotcha, I'd forgotten about the "first time" wording.

That does make it kind of hilarious, though, Unity playing whack-a-mole as they pop up again and being just stubborn enough to make them stop.

What if Smoke Bombs are out, Visionary's Decoy Projection is the lowest HP hero target, and Visionary is the highest HP Hero target? Both cards say "Whenever X would take damage," so neither are once per turn. So wouldn't that mean if a villain target hits either, it will just keep being redirected between the two of them?

Viz takes the damage unfortunately.   That loop can happen in the app currently and you end up with Viz taking the damage. 

This is because each redirection can really only occur once.  So, first the damage is redirected to the decoy projection, then it is redirected to Visionary (but reduced by 1).  If it was damage to the Decoy Projection instead, it would go to Visionary (from Smoke Bombs) and then to the Decoy Projection, but reduced by 1.