Yeah, we could use psychic protection to selectively shield people. Or if we inverted it, we might use it to include instead of exclude. That was kind of my idea For filter intrusion, too - using it inverted to try to invite people instead instead of intrude. Or maybe like you said, by leaving backdoors - so you're the one using filter intrusion on your own conversation, only for other people to get through.

You need filter intrusion to get to a bunch of the higher level skills - like memory immersion, or infiltrator knowledge or persona.

Yeah, you might be able to figure out eavesdropping that way - though it's usually not actually forcing your way through. You're able to see and hear what they do, but I've always done it with permission - I think it'd be much harder to do, otherwise.