What... What? Seriously my brain is now all derpy!

I believe Yukari is involved.

It's possible however... When Yukari is involved, the gap (with ribbons) must have to be there!

Maybe the joke is being over analyzed and all youmu did was "go to the bathroom" as was yuyukos request. No odd portal theory stuff or anything.

But she didn't even pull down her panties, unless she is going commando.

Or even after died, maybe it's ghost body still kept natural urges because, despite being the ghost, she does eat and drink somehow...