If you have Haxchi you can use anything homebrew related just fine. Trying to boot the homebrew launcher or anything else through the web browser doesn't work.
Doubt it walmart has them already unless someone gets a disc copy of botw on wii u. It will not be leaked early even if you happened to have the title key. Nintendo won this one.
I think its going to be a bit more low end for the wii u (not saying the wii u cant handle it). For the switch I am sure its going to be the superior version for BoTW
I think botw might be using some tressfx for sure for like you said grass and what not. Its a strong maybe but who knows would be cool to see it in botw. For the wii u I don't see that being a thing.
BoTW is rumored to be 13.4 GB from what I was told not sure if that is the switch version or not but it shouldn't matter. Assume its 13.4 GB and be sure to have room. Unlike everyone I am going to pick it up for the wii u get a hard copy if at all possible.