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Not answering directly to your problem, but at least the commerce manager menu used to be restricted with access to Admin/SitePlugin, see oldish post: https://swapcode.wordpress.com/2013/10/05/make-the-commerce-manager-link-visible-to-a-group-in-episerver-edit-view/
So based on that post you could add different roles to be allowed to access the commerce manager menu item.
I have tried to add the location path=admin/siteplugin and some other stuff but nothing works?
When i run multiplex and create a user of my own wich i add to WebAdmins group and login (using Forms auth)
Everything works fine.
But!!!
as soon as i switch over to windows auth and windows providers
I cannot see commerce nor addon tabs.
The AD user that i am login in with via the windows domain controller
Is in a Admin group that i also assigned to mapped roles (web.config) to WebAdmins group
Also created my own mapped group and given it full rights
Any suggestion to what i can be missing here?