Ernie, I think iimig stated the main zone would need to be in all channel stereo because if you just took left & right out of the 5.1 or 7.1 pre-outs and put them into Zone2, you'd be missing all the dialog (center channel). 

Putting the main zone into all channel stereo solves this problem, but in my case the main zone will always by in surround sound mode for video sources.

I had a Niles 2-channel speaker to line level adapter that I did some testing with, but it just wasn't suitable because all the center channel info was missing in Zone2.  Some searching lead me to the Audio Control piece.

I'm not aware of a line-level combiner, but as you mentioned any mixer could provide this functionality.  Several manufactures make 1U mixers that would do the job.

If you mix the line level out of the Main Zone pre-outs and then feed back into an input for Zone2 you've solved the audio delay and source tracking issues, but you'll still have independent Zone2 volume which might be desirable for some situations.

For this job using the speaker level combiner does the trick by automatically keeping the volume in both zones synced which is ideal.  It's such a simple thing, but just being able to press volume up/down/mute on the remote and have both zones respond is really cool.

This is one of those silly CI things that a customer could never imagine how difficult it is to actually achieve.  I'm not even sure why something like this is still so difficult to accomplish...