What you're looking for is "triggered character ability" vs. "character ability."
Take Bloodrider -- he has a triggered effect that is NOT a triggered character ability because it is triggered from out of play. However, his cancel is a character ability because he's in play when the cancel occurs. The reason why Bloodrider can't cancel Bloodrider though is because Bloodrider can only cancel triggered character abilities, not simply character abilities. (We agree that he couldn't cancel a character ability like Ser Jorah Mormont's right?)
It's the same with Meera Reed. Her triggered effect is not a triggered character ability because it is triggered from out of play. When her "then" effect resolves, however, she is in play so it's considered a character ability.
You can have character abilities within triggered effects that are not triggered character abilities, which is the case with Meera Reed, Bloodrider, and other cards. I'm pretty sure that is the distinction you're looking for: triggered character ability and character ability .
And for some further clarification: triggered effects and character abilities are NOT necessarily the same. Take for instance, if Meera's then effect were a separate passive (not part of her triggered effect) that happened after she came out of the Shadows. TRV would be immune to it because it's a character ability; Joffrey, however, would not be immune to it because it's not part of a triggered effect.
). I definitely understand the notion, "I don't know how my pie cooks to perfection when I put in the oven for 20 minutes at 350 F,"(by the way, have no idea if that's how you make pie
) but you don't need to know the chemistry behind cooking pie to know and understand what to do when cooking pie. It's the same with the rules. If you look at them you should see Meera Reed resolves the way she does in all those scenarios.