Sounds like the first thing you need to invest in is a good downhaul crank.
Using your back or legs to downhaul a sail, when there are "tools" that take
all the pressure off your body, makes no sense to me. I use my Chinook
Rig Winch even on the small 2.5-5.0 Retro Ripper Trainer sails.
Not sure if a short RDM can turn over a Rio M, but in any case the SDM will not sink
completely, due to the buoyancy of the air inside.
Hope this helps,