The key switch in this car is about the best there is, without going to a mechanical F&R like EZ Co & Club Car use (or the older Harleys). You said you were "warming up" in reverse? Does this mean you had it just sitting there "idling" in reverse? If so, this is a very bad idea.
First of all, these engines are designed for start/stop operation and will warm up as you drive. Also, at idle, the battery is being heavily discharged since it is not really running. It is spinning because the starter is spinning the engine, not because the engine is actually running. This would have drawn a really excessive amount of power through the key switch, over too long a period of time (cooking the contacts internally).
As for the diodes, if the one on the key switch was bad, then it would spin over in reverse but wouldn't start. Is this the problem you have? BU should be able to get these diodes for you, there pretty cheap.
Finally, there was a version that had solenoids for the F&R function. In 1988 & 89 there was the "5 solenoid system" that Columbia used, but it was a nightmare to diagnose when there were problems...
Good luck! Let me know if I can help.