Thank you all for the great replies! I have been leaning towards moving the drum to a thumb shifter, but wanted to ride the bike as-is a while to form my own opinions. I am fairly sure I can skid with the rim brakes if I tried hard enough, but I also suspect that it might be easier with some fresh pads (the existing Shimano pads are hardly worn, but are likely very ancient) which would sway me even more to modernize the brake lever setup. I think giving the control to the stoker also makes good sense. We communicate well, and yes, I would prefer to keep both hands on the bars focused on control while descending. Very good tips on braking as well. Threshold braking certainly is the quickest way to safely stop. All this braking info is very important since even the flattest of rides in our area involve hills, and my wife seems to love to climb (not me, I'm a sprinter, not a climber).

Rick, we'll look for you as well. We'll be on the old blue Santana, no doubt somewhat dazed and out of our element. It will be Lena's first organized ride ever, and my first since the mid-1970s (but at least I think I know what we're in for, in 1973 I was one of the youngest riders to complete the Davis Double at 15 years old and did several century rides back then as well). We plan to attempt a good chunk of the route (~50 miles) this Sunday and by May 20 we hope to be ready to enjoy the ride.

Thank you all again for your replies!