Still not buying that Tenshi isn't involved. Like I said, too many coincidences, and it lines up too well with what she was saying. I'm more saying that she had help/had it suggested by someone else.
Another possible hint is the mention of Shinki not being fond of the Heavens, though I didn't really follow Ran Quest so I don't know the likelihood of that. The main people connected to Makai now are Alice and the Myouren crew... that makes me think Nue, but Tenshi was confused when mentioning that earlier... unless she was disguised as someone else when she made the suggestion?
And as any modern culture of fear can attest, a disaster can be readily used to bolster faith; I wouldn't put the Moriyas out of the running yet, though admittedly I'm doubtful. It's also possible one of the less scrupulous Myouren people arranged it for Byakuren's sake without informing her of it since she'd hate it... but that doesn't seem their style, either - even Nazrin acting on her own.
Not seeing Yuyuko. She has enough to goof off with without bringing the heavens down, and no clear motive for anything past amusement and being inscrutable. Unless Yukari put her up to it, which sounds more likely. (But she didn't respond earlier! - she can be subtle and deceptive when she wants to, I assure you.)
Though, I have another class of suspects as well - The Moon. They'd even share the "bringing heaven down (figuratively)" motive Tenshi has! Just, it'd be more "bringing down those upstarts that think they can be pure while still on Earth, HAH" instead of "bringing down the hypocrites that won't let me have fun in Gensokyo." And one way or another, they'd have the power to do so. As for why now, or why bother, I don't know. Maybe Rise is getting overzealous about her master's intents and beliefs and doing insane things without getting orders first? Though, that wouldn't be possible unless she stole the Gems. And why leave it hovering just over Gensokyo, instead of destroying both outright? That doesn't make sense, either...