justcrankn

You're bringing up many issues which take time to work out. The HRM isn't of much value until you work out the bike fit and cadence issues.
If you're wobbly on the bike there are at least two possible problems. The stoker needs to stay "quite" or the captain will be doing extra work to counteract any movements. Also, the bike handling is probably quite a bit different than your single. If you look at the headtube angle and the fork rake you'll see the difference. The fork rake can be modified with a different fork, but your older bike might be hard to change. Personally I am wobbly with a 54mm fork and we can ride no-hands with a 44mm fork. YMMV
This is debated, but I feel you should use whatever crank length you feel is best for you. Don't modify that for the tandem.
Many women need their seats way back. This has something to do with femur/leg length. This was the case for Miguel Indurain, so you have fast company. This positioning requires a slower cadence to make power. My stoker has her seat all the way back on a setback post. We brought her cadence up from 70 to the 80's and I slowed down from the high 90's. Any faster and her feet are just along for the ride, but I can adjust to any cadence from 75 to 105. With her putting out more power both our speed and endurance are up.
Hope this helps.