I also am working on a TMG-15 with a code 66. I have spent several hours checking power cables fuses and other items mentioned under code 66. Any info that would aid in determining the condition of the controllers themselves would be greatly apreciated.

Code 51 in my experience has usually been a power draw on startup. For instance, customer has a widget installed and it is wired to stay on all the time so data is not lost if the operator turns off the key. Line contacot can be pushed in by hand and truck then operates fine. Power off the widget before startup and code 51 never appears. Now with the other two codes coming up also it would strongly suggest a bad controller as you already know. I would pull out my battery and give a very thorough inspection to all of the cables running under the battery tray. They are prone to corrosion, and short out between each other and to the frame. I think that the 2/0 cable is about $6 a foot now but it is cheaper then a controller. If I can find it I have a list of specs for reading your controller with a multimeter which can help you to know for sure if it's a controller.

Now that I have rambled, does the code come up on startup or after you move the F/R lever.

you can not bypass the controller, but I would not consider purchasing a new controller (which seems to be the price point you are suggesting). There are a few controller rebuilders that will take your controller and rebuild it considerably less than 1500.00 USD, but still not (in my humble opinion) cheap. try FSIP.biz or uniparts.com to see if they may be able to get you going for a lot less money.