Kogasa needs to surprise people not just cause she likes it, but because that's how she eats. I"m not sure if she'll go so far as starve to DEATH if she doesn't, though. This is from the Sanae A route
Kogasa: Well, there are youkai that eat meat,
and there are youkai who eat your spirit.
If you humans don't get surprised by me,
I'll get hungry.
As an aside, there's also this possibly telling bit of dialogue from that route:
Kogasa: Oh, what's the point of youkai who can't surprise people?
Sanae: Well, now, please don't get so down.
Kogasa: Maybe I should go back to being a normal umbrella ...
Sanae: I don't know if anyone would want an old umbrella
that looks like an eggplant ...
Kogasa: That's right, that's how I became an unwanted youkai.
Oh, such terrible old memories ... ...
I always took that to apply to every youkai. In Kyoku's case, if she can't get people to acknowledge her as a youkai instead of just an echo, then she'll become... well, just an echo.
(there's more from Sanae B's route, too. Like Kogasa saying "It's okay. This is how youkai get lonely and disappear." INterestingly enough, on an off-topic note, Kogasa apparently chose to become a youkai so she could get revenge on the human that threw her out, apparently)
Again, I'm not sure why sealed beings are able to do just fine. My guess is that once sealed away and exiled away by humans, it's impossible for them to be EXPLAINED away because their explanation is already there (They're sealed away. That's the explanation). However, that's just a guess. As an aside, I suspect the Scarlet Devil Mansion has separate "arrangements" (Perfect Memento states they live lives that are different from both youkai and humans) but am not sure about that. Even the mountain youkai, despite their imitation of the outside world, still attack humans though (Aya says in BAiJR that she never really saw humans as anything other than prey, and Perfect Memento also states that the tengu's annoying paparazzi activies is not only a way of attacking humans, but one of the most ferocious and feared ways. Of course, this is likely ZUN's sense of humour but it still stands, I think. Aya demonstrates her skill at "attacking" humans pretty well in Oriental Sacred Place when she reduces Reimu to a huge panicing mess over not reading her newspapers)
As for "collective belief", Byakuren's profile did state she had to keep youkai alive cause that's where her power to stay young stemmed from. It's the reason why she helped youkai in the first place (BEFORE she became sincerely sympathetic to them, she was only helping them because without youkai around, her youth magic wouldn't work)
From Byakuren's profile:
The only way to lose that black art was for Humanity to reject it entirely.
In other words, if youkai ceased to exist, she wouldn't be able to sustain her own power.
And so, she honored youkai.
She pretended to be a great youkai exterminator, but actually saved the youkai behind the scenes.
Again, I'm pretty sure Symposium of Post Mysticism is at least in part meant to explore alternative ways of youkai existing without attacking humans.