Here is my take on this:
I think that this concept can be realized with a much simpler design. It is all theories but may be worth a try. The magenta cylinder is a spring applying pressure to the lever that in its turn keeps the belt under tension.
Possible downsides to this whole design is that the filament is bent at the same radius as the large wheel. That may break the filament over time, when the machine is not in use. Maybe it can be overcome with a bigger wheel, the one in these sketches has a diameter of 50 mm. A bigger wheel also gives more grip surface on the filament. It is also possible to have the filament go 180 degrees round the wheel for even more grip, but then again, bending the filament to radius that narrow may cause problems.
In this design it is the belt that drives the filament and the big wheel is the support, the design in the first post it is the other way around. So a o-ring in the groove of the big wheel should not be needed.