the sub-command npm prefix logs /usr/local and that's also where it tries to make global installs (obviously raising an EACCES error).
for other things, options are correctly resolved eg. npm config <command> -g will access the corect configs
global installs going to the folder specified in prefix
npm i <@some/package> -g
should have been fixed in v7.0.0-beta.11 please let us know if you get a chance to try it again 馃槉
@ruyadorno looks like it works.
Now I finally don't have to use npm exec npm install <pkg> -g anymore. It looked especially ridiculous when installing the subsequent npm-beta version.