"DIY longtail, Could it be this easy?"
I think Dogman and I are the biggest supporters of full-suspension longtails around here. He rides his often, and mine has been dis-assembled since I moved to another state (only have a one-car garage right now).
While mine was running, I loved it. I had it adjusted so that my feet were flat on the ground when I was at a stop. With fat tires it ran like a dream floating on a cloud.
Using a wheel bracket as the shock-mount is brilliant.
The flat plate that's anchored to the V-brake bosses is a great idea (mine was a disc-brake frame, no V-brake bosses). Such a plate would allow for several mounting-hole sets, so the shock could be moved forward or back (depending on the length of the shocks you experiment with). If the cheap-o WalJunk shocks were too craptastic, a more-expensive air-shock could dramatically improve the shock performance, with your budget being the only restriction.
I planned on a 24-inch (or 20-inch wheel with fattie moped tire= 22-inches, very flat-resistant) on the rear wheel for a future build to make the bike a little shorter overall. I wanted to show that it was possible to do with just a hacksaw and a drill. I put it together while I was renting a bedroom in a 2-bedroom apartment. The bottom pic is the money-shot of Dogmans frame-set. By leaving the top-tube on the rear frame section still attached, it provided a perfect place to hang cargo boxes.
Between your method and Dogmans, making a FS longtail is very easy and cheap, I highly recommend having a second E-bike like this as a back-up. Sooooo cheap to put one together.
"Full-Suspension Longtail 2.0"