I deliberately parted two joints on my Quando frame to aid the rear frame expansion, and also reshaped the frame to bring the dropouts back to parallel, then re-welded the joints. From what I can see of your Reach frame it's not suited to this treatment and I don't think you should attempt the 15 mm expansion.
The 20" wheel eZee Quando originally had a single speed freewheel on it's powerful motor, giving a gear of just under 70". Coupled with the powerful eZee motor that would be ok for me to climb a 14% hill easily, while being able to keep up with the motor to 15 mph at a high 85 cadence.
With your chainring shift at the front as well, a single speed freewheel is certainly practical, but you may need to use a wider than usual chain or grind the freewheel teeth thinner.
P.S. You asked above if an offset rim is a problem. So long as you can achieve correct rim brake adjustment, no it's not. Some bikes are even designed that way, the Burrows 8freight being a notable example with the whole rear wheel tracking well to one side of the front, though that does have a long wheelbase.
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