Hmmm. Before any comments directly to the subject of e-assist. I have to observe that hills are simply a matter of gearing! Unless I way miss my guess that tandem has a low gear around 28". If instead it was 18"! Maybe even 16"! Would the o.p.'s knees still be in distress? Also any e-assist will work better when the underlying gearing is optimized for the riders strength level.
A 20" powered wheel is a torque monster. A tandem will need no less than a 500W motor and that simply is too much torque (power) to put at the front end. Looking at that bike, I would get rid of any IPS, if present and put a mid-drive on the Stokers (1st choice) or Captains (2nd choice) cranks and run the wires where they have to be run. There will be plenty of placement options for the battery pack(s) to optimize weight distribution. It need not be confusing. I am ready and willing to DM about this, and if you hit up Eco-Cycle who mainly deal with Tong-Shen TSDZ mid-drive systems, then you can talk with David Hall about these motors as well. Bafang mid-drives are powerful but they are not torque sensing. I went with the Tong-shen for that reason. If I had it to do over I probably would just go Bafang. Cadence sensing isn't the worst thing in the world, especially if you are so ... *cough* unacquainted with e-assist systems, as to be unaware of the various technologies and what they represent.
''If you are NOT going to install the system yourself then, IMO you should find a dealer who can and trust them on what to install and how* but, of course, it helps to have some knowledge of what you want. There is much more to it.
*make sure they pay attention to providing adequate torque arms, ideally on both sides if a hub motor. Tong-Shen mounting kit for recumbents has a nifty torque arm for boom mounting (Captains cranks) the motor.