Not possible to separate "can edit" and "grant edit"

It is not possible to separate concerns this way, e.g.:
Taxonomy terms:
Firm
- Department A
- Department B
- Department C

Let's say I want to create a node that is in Section "Department A", so everyone who has access by role to edit "Department A" nodes, can do this.
Now I want to create a node that can be edited by anyone in the firm, so I put it in Section "Firm". For that I have to have access to section "Firm" too by role, otherwise I can't see the Section "Firm" in my select widget.

What is the problem here is that if I have access to "Firm", I automatically have access to "Department B" as well. This isa security problem.
I can not give users the possibility to put content into a section where it can be edited by others BUT am NOT allowed to edit other sub-items of that parent.

Solution would be: automatically see (and be able to select) all parent sections of the ones that I have access to. But not be able to put something into other subnodes of that.