Due to the chronic lack of documentation for CB, making sense of the plugins is near impossible.

Seams a bit unfair when talking about CB Auto Actions. There's nothing to document for CB Auto Actions. It's just a tool of which we've provided some examples all over the forums and here . That's like documenting what to do with a hammer. As for how to use it it sets all of that up for you (trigger > type > conditions > action). Suggestions are always welcome, but criticism without feedback is just negativity that benefits no one.

I've tried various triggers and conditions but nothing appears to work.

You're wanting to execute a trigger after a user updates their profile. This makes the trigger quite clear: onAfterUserUpdate. All of our triggers are setup this way with a few legacy triggers being an exception. We try to name them so it's obvious when they're fired. As for conditions you need to use actual substitutions to condition against a value. Examples as follows.

Incorrect
cb_mid_a Equal To 0

Correct
[cb_mid_a] Equal To 0

The first one you're literally matching the string cb_mid_a against 0 which will always fail. The second one you're matching the substituted field value. The below resources can help with both of your issues.

List of triggers and their variables
www.joomlapolis.com/documentation/279-community-builder/tutorials/18358-using-cb-triggers

Substitution usage information
www.joomlapolis.com/documentation/279-community-builder/tutorials/18353-using-substitutions-throughout-cb

Correct, you were updating the user from backend which is a different trigger from frontend.

Frontend: onAfterUserUpdate
Backend: onAfterUpdateUser

Maybe I am being a bit unfair but you have just provided two links which are not to be found anywhere in the auto actions documentation. As a positive suggestion it would be useful if those links appeared in the auto actions documentation some where.

They're not in any way CB Auto Actions specific. They're specific to CB it self so are in CBs documentation. We're also here to help and highly recommend instead of sitting in frustration simply ask as we've no issues providing guidance and even simple examples. With that said we still have a ways to go to improving our documentation, but time is a huge limiting factor.