Permissions inheritance for private, unlisted groups and subgroups

I'm building a new group in our community. The group has a few subgroups, and in each subgroup, a few applications. I need to keep the group completely private and hidden until go-live, so I created both the group and the subgroups as Private, Unlisted.

I have a couple of colleagues helping me seed content, so I made them Managers at the group level. I noticed, though, that making them Managers at the group level didn't cascade down to the subgroups. In order to give them the permissions they needed, I found that I had to make them Managers at the subgroup level, too.

My question is: is this expected? If so, is there another way to do this which wouldn't necessitate me having to add permissions twice (to group and then subgroup)?

Once we're at go-live, my plan is to just switch the group and subgroups to the desired type: joinless. Does this plan affect the answer to the above question in any way?