List all Authors in active Category & link to their posts in that Category only

Hi.

I have some trouble getting my mind around something and hope that one of you guys can help me out.

I’m doing a site for a small arts collective and it’s up and running on: https://www.lula.se

Now the way the site works now, it has a bunch of different Authors (listed with their full name in the left sidebar) and 5 categories (listed in the horizontal menu, plus the Contact page).
The horizontal menu uses “wp_list_categories” to fetch the categories which works fine, and when you select one all posts in that category are displayed (one by one with WP-PageNavi as navigation).

My problem is this:
When you are in a category, my client would like the list of names on the left to change and only display the names of Authors active in that category. But not only that, they would like the name to link to posts by that author, only in the currently active category.
I’ve been reading up on tons of variations of how this could be done, as well as tried numerous of my own ideas, but I’ve failed to find a solution that works while giving my client full control in adding new categories and new authors without my help.

Please ask me if I have not been clear enough about something!

If anyone would like to give me some pointers, that would be much appreciated. I can’t offer any monetary compensation, but if someone could figure this out I’ll be glad to help with any design related issues.

Thanks!