Sounds like a real tale of woe. There are several clues as to why reverse would not engage hidden in the correspondence above.
First, does reverse engage in the main box but will not drive the car ? If so, the shift rail is not locking the overdrive unit out.
In this case, when the overdrive is manually locked out, reverse should work.
Second possibility is that the solenoid is energized so reverse cannot be selected. In this case, when electrics are disconnected, reverse will work.
Third is if the shift rail for low is not engaging the overdrive lockout rail there is either a part missing or some internal interference due perhaps to the front case not being the one designed for use with overdrive although the only difference I have found is an oil delivery passage.
What you need to do is pick up your overdrive and RUN, don't walk and find yourself a mechanic who is willing to read the shop manual. These are simple mechanical devices and EASY to service. I am in Canada, but a long way from you otherwise I would show you how to do it and guarantee it will work.