You cannot select more than one category. So there is no easy way. But still:
Create a copy of "GroupA" (on the same level) and name it "GroupBChildofA". Then remove A,B,C from "GroupA". Faster than copying every category individually.
Setup a view filter which hides all images and apply that to the thumbnail window in the categroy view. This way IMatch does not need to always reload the thumbnail window contents and this speeds up everything considerably.
For example, create a filter with a file name mask of something like
which will never find anything. Store this filter as "Hide All" and then make this filter active before you start working in the category view.
IMatch 5 has a dedicated feature for this purpose
And, you can select multiple categories...