Torque effectiness

You need Garmin power-meter pedals.
www.garmin.com/.../

I will need a power meter that supports the ant+ power profile which since 2012 have had support for the PS and TE ( Pedal Smoothness and Torque ) 

This doesn't mean that it have to be a garmin power meter pedals.. Other PM have provides this data as well. I have dual rotor pm from 2015 and I get L/R balance, TS and TE.

It might be that you PM simple doesn't support these two ant+ power profile metrix PS,TE but does L/R. The reason for this I do not known. A limitation of the PM or something else. 

However despite having a PM that provides both PS and TE I have not used the values for anything. Despite having a left leg that have been broken and been through rehabilitation, the L/R balance was all I used. And even now when fully fit don't pay that much attention to that either.

Yep.  You'll need a dual sided power meter that provides that data.  I've been using Assioma Duo power meter pedals for some years and it was interesting to see that data, but it wasn't something actionable in my case.

I have the XC200, and the BALANCE % showing a reading seems to suggest that they are working.

Are you indicating that maybe only 1 of the two pedals is being read?
Or is it possible to get BALANCE with only a single pedal?

i believe you need dual sided pms and they have to be ant+. my stages sb20 which has  dual sided PMs and is paired via ant+ shows these metrics just fine

How do I know if I only 1 side is being read?

Do you show the pair of pedals connected as 1 pedal or as 2 connections?

My Favero power meter pedals must have  this turned on in the sensor settings. The power meters must be activated and "on" or the on/off button  will not be visible.