Where to stay along Rhine or Moselle? Cochem?

Where are you arriving from? What time of year is your trip?
I'm not sure I would describe Heidelberg as "on the way" to the Frankfurt airport from the Mosel Valley.
There is plenty to see and do in this area, but if you really want to stop off somewhere, how about one of the spa towns like Bad Ems? Could help you recover from your light walking, etc.
The whole of the Mosel Valley is very beautiful, and it's dotted with charming villages (nearly all of which have wineries). I suggest using your favorite booking site and setting the map to cover the whole area, set the filters for your needs and budget, and see what you find. Take a look at the village of Starkenburg, which sits up on the hill and has beautiful views of the river valley.

2 nights, a half day on Friday, a full day on Saturday, and another half day on Sunday. One full day and 2, at best, half days.

In 2004, I spent 5 nights, 4 whole days + 2 partial days, on the Rhein based in Boppard. I saw the Marksburg, St Goar (Rheinfels), Bacharach, Oberwesel, Koblenz, and the Boppard area.

In 2008, I spent 4 nights, 3 whole day + 2 partial days in a town on the Mosel near Cochem. I saw Cochem, Bernkastel, Zell, and Beilstein. Burg Eltz was closed for the season when I was on the Mosel.

I spent 9 nights, 7 whole days + 4 partial day, 10 venues. There's plenty to see see in the Rhein/Mosel area. You only have 1½ days. You'll get to Cochem too late on Friday to do much more than have dinner and briefly see the Cochem, probably not enough time to tour the Reichsburg. The best thing to see in that area, IMO, is Burg Eltz, but that will pretty much chew up Saturday. With a car, you might get to see Beilstein on Sunday. It's really small; doesn't take more than a couple of hours to see it. Or you might stop in St Goar (Rheinfels) on the ways to FRA.

I've been to Heidelberg twice. I wouldn't waste time going that far out of the way. I'd go to Mainz or Frankfurt (my choice would be Mainz) the last day before flying out.

We spent 2 nights in Belstein on the Mosselle, it was small but delightful. There is a larger town across the river . You could visit the area from Cochem. Cochem would be a much larger base. We also stayed in Baccharach, but did not find it very charming. it was 2021, and I don't think they had recovered from the pandemic yet. You are not going to have a lot of time.

Lots of great ideas so far. We did spend time in Baccharach and enjoyed fine wine, good food, and light walking on empty roads out of valley. Great views of the river there also. On our walk down one afternoon this woman stop us and had us sit down with her and offered us a glass of wine each. What a local experience for us. She knew little English and my wife knows a little German so we had a great time.
If you had more time: I would tell you to take the train to Trier. We rented bikes and covered a lot of ground. We even ended up in Luxemberg on one ride. Some Roman ruins there also you can explore.
Have fun.