This is a further follow up, which I share because it might help others with similar problems, now and in the future.
Made further progress effectuating the exchange. Returned the PS 3 steamer with drawbar problem to dealer, who received it, and who is shipping a Proto 3 diesel to me, which I should receive next week.
Moral of story: buy from a reputable fair minded dealer (my experience with Forum Sponsors so far is that they are all reputable and fair-minded) and take action while engine is still under warranty.
This morning, I ran an MTH PS3 diesel on DCS, strictly following the crystal clear instructions in the DCS Operator's Manual. It ran great. Here is the PS3 CSX diesel hauling gondolas in the morning light:
There is absolutely nothing wrong with my brand new DCS Remote Control Set with TIU and handheld remote unit, and the way I set it up. Much of the confusion in my prior posts was due to the fact that I initially did not know if the problem was due to the DCS, the steamer, or both.
Incidentally, I set up the DCS with the variable voltage channels functioning like fixed voltage channels, which is one of the options recommended in Barry's book. This works for me because I am fine running Postwar and PS 1 engines from the transformer throttles, because I can still run PS 2 and 3 engines and LC+ engines from their respective handhelds to do switching maneuvers.
My problem was caused by the PS3 steamer and its drawbar.
One final thought: i plan to minimize problems with tethers and drawbars between steamers and their tenders by rarely, if ever, disconnecting them. Whenever I put one on, or take one off, the track, I plan to do so as one unit, if possible, holding the engine in one hand and the tender in the other. Arnold