I think the interactions themself are extremly powerful. But since the deck doesn't run any tutors, it is important that every card is worth it on its own and/or has synergies with enough cards in the deck. For the cost reduction spells, I am sceptical whether they are solid enough outside of the combo. Both are at worst bad mana rocks with potential card disadvantage. The upside however is that you can unload a full post-Kozilek hand faster. These have to be tested to see how they perform.

With Rings of Brighthearth the same point as with the cost reduction artifacts can be made, but I think the card has almost no utility in the deck, making it a dead card outside the combo. In this vversion of the deck, I wouldn't play it. If you want the deck to move in a more volatile direction, you can add all sorts of, overlaping, combos to try to get some infinite stuff working. This is an option, but would require quite the rebuild imo. Not of This World actually already is in the deck :-) and it's extremly good in this deck.

I agree with each card needing to be useful on it's own due to the lack of tutors. Rings might have been a stretch, but cloudkey is the best pick of the lot. It doesn't give you card disadvantage and is flexible as "another rock" as you mentioned while at best giving you a combo

Still curious how often Manifold key's unblockable ability is used, because if not often, I would swap voltaic key simply for the ability to do the combo should the pieces come together.

Good Point on the "Keys". On the face of it, Cloud Key looks amazing because he let's Kozilek 2-hit people all day. But I (as i am the player i the other side of this deck most of the time) don't recall this ever being a thing. this means Voltaic Key is strictly better, allthough without Rings of Brighthearth I also don't see any utility for the ability to untap the Key itself. Do you see any in the deck? One could also make the argument to just play both, if there are more Rocks in the deck that produce at least 2 mana, with Doubling Cube and Grim Monolith being possible additions to the deck to provide consistent nut draws.