There were clamp-on downtube shifters in the past but,
AFAIK, they were only made to fit "standard diameter" (i.e. 1-1/8") downtubes and your Randonee very likely has a larger diameter downtube so that isn't an option.
One way to get friction shifting is to fit bar-end shifters. They all provide friction-only front shifting and even the indexed rear shifters have a friction option.
As to the rear derailleur hitting the spokes, either 1) the low limit screw has to be readjusted or 2)the derailleur hanger is mis-aligned or 3) the wheel dish is incorrect.