With the age of this lift having a crown technican may be the best. Check out this link to find a local dealer: https://www.crown.com/en-us/forklift-dealers.html?countryCode=US®ion=TX&latitude=31.9685988&longitude=-99.9018131

The contactor may not spark when closing. That only would happen if there is a large load in the circuit, like trying to run the drive motor.

Even if you got a little water on the electrical parts it could cause them to short. If the battery was disconnected from the truck it would be less likely but still possible to short something out.

Negative values just means you swapped your positive and negative lead on you dvom. You can just ignore the negative mark when testing. If this is an electrical issue, I would place your negative lead on the negative battery cable and start testing for voltage at the drive motor terminals and work your way back to battery positive testing all the components in that circuit. This will show you were you loose voltage.

This is definitely something that is difficult to diagnose as there could be lots of issues in the power circuit or even the control circuit. My best recommendation is to have a technican come out. It should not be a difficult fix for an experienced technican as I deal with these issues all the time. The only issue is they are diffrent from truck to truck.