The washer deffinately works. I use a flat washer straight from the hardware.
You may have to spread the chain stays slightly to fit everything in, but it's no big deal, I can do it by hand or sometimes a little careful prying w/ a flat bladed screwdriver.
I use a 7-speed free wheel w/ a 9-speed chain and derailleur and it shifts fine, but I only use 1 or 2 gears, maybe 3 at the most and if you have any kind of hub motor, you won't shift it any more than that either. Even on my "assist bike" w/ it tiny Q100 hub motor, I only use 2 or 3 gears. So I just adjust the derailleur to optimize the shifting on the lower three gears.
Speaking of adjusting the derailleur, I assume you are using the DNP free wheel because it's the only one w/ an 11 T sm. gear. I have found that because of the shape of the teeth(shark fin shape), shifting can be improved by moving the derailleur up higher and further forward than it's normal position.
'03 Rocky Mountain Edge 2WD 260 Q100H frt and Ezee V1 rear 2 Elifebike 20A & 25A 9-FET controllers 12S/20Ah Multistar Lipo rear 5Ah Turnigy frt Luna Cyclops Extra lite Alex 24DM rims, Crazy Bobs run ghetto tubeless. 25 mph. Mean Well HLG-320H-54A
'07 GT Idive 4 4.0, Q100C 201 14S LiPoly elifebike 9-FET 20A controller. 23 MPH.