Re: Including groups in groups
The main purpose of including groups in groups is so that permissions assigned to the included group can be shared by the including group and so don't have to be specifically assigned to the including group. See this Doc page.
For example, any permissions given to your Group_A and Group_B will be shared by Group_C. From the docs page it appears only global permissions (set on the Group Admin page) are inherited, not specific-object permissions. As always with permissions, it's good to test everything by logging in as the various users and making sure accesses are working as intended.
-- Gary - zukakakina.com