Update:

It seems that oldbobcat was right on the money with this issue. I purchased a Park Tool, dérailleur alignment tool, and found that even though I had a new dérailleur hanger, the dérailleur needed some significant alignment changes.

Now if I back-pedal, the chain will only come off the outside of the big crankgear (50-tooth) if I'm on my smallest sprocket. All others are fine.

In addition, when I'm on the smaller crank gear (34-tooth), I can run on the inboard 9-sprockets, and only get some rubbing on the 10th when I have significant cross-chaining.

So overall the problem has been fixed !!

Thank you all for the help along the way,

Terry