How can I let a group of users edit anything in a specific section? Our content editor wants to let the various authors have rights to edit things in only specific sections, and it will often be multiple people assigned to a given section.  What's the best way to do this?   (And sections do NOT map neatly to content-types.  They're based on the menu/site structure.)
It's not clear to me what your rule for determining access control is. I'm going to guess it's menu paths.

I had a similar problem recently. I needed to ensure that users trying to access /somePathName/* were of a certain role. This is a bit of an odd scenario since any path in the system could be aliased under one of my UAC paths and it will automatically become access controlled whether it was a term, node, user, whatever. 

I looked into existing solutions and found none. I wrote my own module that managed access control based on "arg(1)" and a user's roles. 

Access control in Drupal is one of those pieces that you understand after a few hours of studying, do the work, then completely forget how you did it. Or maybe that's just me.

Documentation on Drupal.org is pretty good on this topic. Be sure to install the Devel module and enable the node access modules it provides. They are very useful.