oh yes i remember these trucks well...
they can be a headache at times for sure

from the sounds of your description of what the truck is doing it sounds like the truck software is corrupted.
I've seen a myriad of issues dealing with this problem and it was mostly due to the operator plugging the charger into the truck instead of the battery. I went round and round with Yale and Schaeff both on this problem with a customer of mine until they finally came out with what they called a "crowbar fix" which consisted of installing some components inline with the battery circuit to help aid in discharging the system and prevent any leaking voltage that might trigger a charger to turn on if by chance it was inadvertantly plugged into the truck instead of the battery. In every case i had where the display got corrupted or damaged like this it was always replacing the logic unit in the display to fix it. (basically replacing the display unit) Back then it ran about $1200.00 for a display.
Since you are getting it to power up you might can run the software/firmware upgrade and see if that will repair any problems causing this.
This update does not require any special canbox tool, just a null modem cable and a laptop capable of running dos (older version of windows (XP or earlier) with a db9 serial port. The software i have comes on a floppy disk. I dont think a usb serial adapter will work but i'm not sure, i've never had to try that method as i still have an ancient laptop that can run this ;o)

As far as that key problem it might not be a bad idea to get a new keyswitch. Try this first before attempting to upgrade the software but i'd see if you can see what version software the truck has when you turn on the key, it should pop up at first before it does anything else. Then see if there is a newer version than whats in it and get that and try to update it after replacing the keyswitch.

good luck