Uh, yes they do, they are both native to it. More generally, that's the entire theme of the plane.Neither orgies nor maenads have any inherent connection to Arborea.
If you are renaming the Beastlands to Yggdrasil, which I'm not necessarily against, what are you doing with the canonical Yggdrasil and its role as a planar pathway? Because I always thought Yggdrasil (and the Endless Staircase, but that's a different matter) were kind of the coolest way to travel the planes.
Also, if you are keeping Ysgard (as Asgard) and are renaming Acheron to Valhalla, what exactly is the difference between them? Also, the theme of Acheron that made it so lawful was that it was about following orders, the destructive hopelessness of war and the dehumanizing effect of fighting in vast armies. Any kind of Valhalla would almost certainly be about personal glory and individual combat, how do you make that lawful in philosophy?
And if you reduce every plane to one climate zone, don't you remove some of the most interesting layers of hte planes? For example, I was always extremely fond of the dark crystal caves and shadowy mysteries of the lower layers of Ysgard, or the whispering deserts full of secrets of Arborea. How are you incorporating such interesting, but unusual places?