know that he's okay... otherwise leave him alone.. moving the decor around, and otherwise fitzing with things ime encourages them to find better hide spots, which are usually more dangerous. If your spiney is MIA for 3 weeks after a 25% WC there are more issues to addess then skitishness. That is not normal behavior of a spiney thats "happy" with it's tank. If you don't have driftwood in the tank make sure to add some, particularly young ones I've had find them safe havens. By not using the hide spots you've made available your spiney is making it very clear none are acceptable. imo you made a good call putting the powerhead back in.. as long as your husband knows not to plug it in. at least until your fire outgrows the powerhead. Once he settles into a spot, I would go to great pains at working to hand-feed him to get him associating you with food.
You've hit one of many hurdles you can run into with keeping these fish. But they are well worth it, as orange pretty much summed up =D