how can Koakuma position her legs like that while Patchouli is sitting there?

Giant, luscious succubus hips?

Reminds me 77gl's art

... Don't actually see any issues, there. Matter of perspective.

Actually, there is; Patchy's left leg seems to be connected to the body right over Koa's thigh, while the righ leg is living a life of its own with Patchy's butt which is almost sitting on the chair.

Whew.