Hey Mike, nice progress.
Just looking at your skeleton there and noticed the hips and femur bones are bending. I know you're focusing on the upper body -- just wanted to check the bones in the upper body are moving as rigid objects, rather than bending.
Could be wrong but it also looks like your scapulae aren't moving.
As for the lats, try putting a zFiber on there. Paint the end points so that the insertion onto the humerus is black and the vertices that run along the spine are white (or vice versa). At the start of your animation, set two keys on the excitation attribute. 0.0 at the start frame and 5 frames later, key it to 0.3. You should see the lat contract over those frames and you'll have tension in the muscle over the course of your animation.
The reason to key it, rather than just set it, is so that the muscle doesn't jump abruptly on the first frame. This will give the collisions a better chance to resolve.
You can still use zLineOfAction to do additional contraction for different poses.
I'll let Lonnie comment on the abs