1) What temp is your fridge?
2) Once you solve this pressure problem, you'll find that 6' is probably too short. I'd suggest 8-10 feet to prevent foamy pours.
As you're discovering, you can't use the regulator to lower the pressure in the keg. The reason the regulator gauge is shooting back up to 20 is because that's what the pressure is in the keg.
Do you have a ball valve on your regulator? I'd disconnect the keg from the gas (if you don't have the ball valve you'll have to un-couple the keg), turn the adjusting screw/knob on the regulator all the way counter clockwise. If the pressure is still reading 20 psi, try pulling the relief valve on your regulator. Once it's at 0 psi, slowly set it back up to 12 psi, then don't touch the knob!
Now when you put the keg back on the gas, the regulator will add CO2 to the keg once the pressure falls below 12 psi. If the gauge reads higher than 12, it means the pressure in the keg is great than 12. If the keg is really overcarbonated (not sure how full it is or how long you had it at 20 psi) you might need to shut off the gas and vent the keg every few hours for the next couple days. And yes, in that case you want to use the relief valve on the coupler.