Downloading games from Wii U USB Helper is illegal, though everyone does it and nothing happens, so it's really a moral problem only.
As you have bought it from the eshop, I think you can download it without problem.
So I tried to copy and paste a level made in Miyamoto! level editor while the game was running in CEMU and it worked.
Just make sure you are on another level or the World Map.
This should make testing levels a lot easier.
I may try the new ram site, and download more RAM to my Switch. Not sure if ddr3 is the right ram
for it tho. Edit- no it uses floppy Ram, just like @AncientBoi