They have one other weapon...if they invent the spell card rules, they have Sakuya's time-stop ability. Though I have no idea if she would mention that or not.
The Youkai Tree incident and it's sealing (with Yuyuko's dead body), happened more than 1000 years ago. And The Great Hakurei Barrier were cast around 900-1000 years later. Not to mention, Hakugyokurou was separated from Gensokyo, until the spring snow incident.
Sakuya really messing around with timeline this time.
I don't think so. Suika's power seems to be based on materiality, so I doubt that she could gather something without any physical form. But, really, who knows?
...and that's what's goin' on.SshzzzSo that's it.To be sure, living while bearing this disgrace......would be more than a human can bear.And this apparition......knew but said nothing, hm?What good would knowing about it have done you?It's got nothing to do with you....we should be able to isolate the youkai cherry tree and the vengeful ghosts......temporarily from this world......until that lot finish off the cherry tree, shouldn't we?...the "Great Hakurei Barrier", right?Nothing to do with me? If you and I work together...It's for times like this that we reserved...