But I can't find a rule anywhere that says I am limited to discarding just 1 per rest action.That's because there is no such rule.
Is there an official ruling on this?I haven't found one. I think no one supposed it would need a clarification.
Any number of effects may trigger on a single event. You may have for example (card names from my memory, so they may be a little off) a Lodge Researcher which says "When you defeat a monster during a Combat Encounter (gain 1 clue)" and "Purifying the World" Which says "When you defeat a monster during a Combat Encounter (put a token on this task)". Do you have doubts that the both effects trigger when you defeat a single monster?
Also the symbol basically means "When a appears on a Mythos card (do something)" Do you have doubts that a whole lot of effects trigger on a single ?
If not, then don't have doubts that a several effects may trigger on a single Rest action. Or any other event.
When something happens, then all the effects that state "When something happens" trigger.
Also, as a curiosity, look at this:
The Poisoned condition states that "When you perform a Rest Action" you don't recover any Health or Sanity. If you should choose which of the effects trigger on a Rest Action, then you would just choose to resolve Paranoia's effect and recover Health/Sanity as normal.