When you are in a subgroup, to return to the main group you have to go up to "Your Groups" in the header and click on the group from there.
That is due to a relatively recent change. Previously clicking the Subgroups button on the left would bring you back to the primary group, and from there you could select a different subgroup or use the other buttons to access primary group pages.
That method was counter-intuitive at first, but handy once learned.
I have a suggestion which I believe will help in the overall user-friendliness of the site.
Although I like your second proposal (nested under Subgroups) better than the original, I think a far simpler approach would be to include a link to the Primary group on the Subgroups page. Perhaps above the Subgroups You Belong To heading, with a heading of its own: Primary Group.
I'd have to try it to be sure, but I don't think I'm a fan of having that nested list of pages open on the left whenever working in a subgroup. Especially as a group Admin, that column would get awkwardly tall I think.