GOT IT!!! Thanks guys for all the tips and commiseration. Might be time for me to start writing checks to have someone else maintain my car. 
What finally worked was a crescent wrench and a breaker bar. Thank goodness the threads on this piece were softer than the threads in the housing. It's not magnetic, so I'm guessing it's aluminum. The threads on the housing were a little damaged, but fortunately I had the proper tap to clean them up - 12mm x 1.50 if anyone cares. Wheel is back on. All five lug bolts seemed to go in and torque down just fine.
I think what happened is that when I first put the wheel on, it kind of hung with all its weight on that thing and started misaligning the threads.
I certainly won't be using that "helper" again and I'm pretty sure I won't be looking hard for a replacement. 
__________________
Len
'59 220S Cabriolet-SOLD and living happily in Malta
'83 240D 351,500 miles original owner-SOLD
'88 560SL 41,000 miles - totaled and parted out
https://sites.google.com/site/mercedesstuff/home
'99 E300 turbo 227,500 miles
'03 SLK320 40,000 miles - gave to my daughter
'14 Smart electric coupe 28,500 miles
'14 Smart electric cabriolet 28,500 miles
'15 Smart electric coupe 28,000 miles
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