Core stability and strength: Does anyone have a means to strengthen the core without putting tension on the neck muscles? I'd like to work on my abs and core, but everything I've seen wouldn't be good for the rest of me.

Diaphragmatic breathing is important. IMO you must have joint mobility and flexion of the ribs before you will see a benefit with deep breathing. You can still breathe deep, but your breath won't be utilizing the capacity of your ribs or pulling ribs down which are stuck. I also feel you must use all ribs to breathe completely. Avoiding the upper ribs will not loosen or get them to articulate as they should and will cause further problems. Breathing into your BP can be beneficial and shouldn't be avoided. The breath expands front, back, and to all sides and having capacity in all directions is something most people do not think about.