I suggest as easiest way of achieving this is to install the add-on that gives more role levels. (multi roles?) it will give you 4 settings per category instead of 2. this way you can have members set to read only and admins set to read and write. hope this helps. im too lazy to search for the addon for you, but im sure u can manage :)
CategoryRoles should do what you ask for, if you add the required delegate and checkout the roles ordering, as stated in Readme. If not report why it does not.
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And CategoryRoles doesn't work either... :(
If not report why it does not.