I used to get severe tendinitis from over mouse use and here are two things that helped me: 1. Swimming laps (physical therapy) 2. Alternating mouse usage between left and right hands (I am right handed and it did not take long to get proficient with my left hand).
The LOGITECH MX VERTICAL MOUSE was built with the idea of using less muscle and configured to a hand movement. Sadly, everyone is still trying to improve a design that was created on a roll-on deodorant model. You can only go so far with that kind of a weak foundation.
OK Gotta chime in here. I have been using the Logitech MX-500 mouse for over 12 years now...with no ill effects. It may be discontinued now, but can probably be found on EBay. It is a gaming mouse and very easy on the wrist. Never had a mechanical problem with it either. Secondly, I learned a long time ago after countless hours of gaming, that in order for ANY mouse to be comfortable with little impact to the tendons in the hand, is to ensure that the elbow is lower than the hand. If your chair is too high, so as to get a better look at your monitors, it may be better to lower the chair and tilt those monitors down a bit. I highly recommend this mouse. 20 Hours a day??? Even a butter bath would get uncomfortable. I hope you find relief Mark.
I have read some stuff about people using a wacom tablet as a mouse, however no personal experience yet. Might try it soon.
There are other "soft" ways to adjust the amount of energy expended when clicking the mouse. Firstly, I do not understand what the OP means by having to "cock the finger so high in the air..." Why would you do that? Just let your finger rest on the mouse button until you need to click. Then, just click it. Don't keep your finger way up in the air in between clicks, would be most of the time. What is the point of doing that? Silly. Second, you can, in Windows OS at least, adjust the time needed to double-click. You can set it so you don't have to make like a machine-gun on the clicky-finger. You can make it a more relaxed double-left-click. Those tips may help you for now.
I had a logitech trackball and loved it. Alas, it just died...and they don't make them any more. There was never any fatigue or aches with that sweet device. Apparently no one liked them. I now have a Logitech Triathlon, but it must be used with a wrist support mouse pad. Try this: $8 bucks....