mech
when testing with the handyman if you run a leak test and that red light comes on at all replace the rec... even a slight blip can mean a leak enough to cause a fault. Remember these systems are pulsing several thousand times a second so it measures time in milliseconds which very easily can translate into too much time for a rec to be on or off when its not supposed to be causing a fault.
Now as a precaution when replacing the #5 rec i would always replace it with the same as the #2 rec because of the amp rating being higher. The #5 rec amp rate is low because it's only function is to turn off the pulse cycle once the cap circuit is charged and ready to fire the #1 rec. So replacing it with a higher rated rec usually solves any future potential issues with that component which i have seen happen in the past.

As for that black box on the parking brake switch circuit that sounds like a relay that may shut off the drive when the brake is applied to keep the truck from driving through the brake, and then again it may just be an alarm relay to tell you when to set the brake if you get off the seat.

As for the plastic contactor under the floor plate?
Not sure what that is, i do not have a manual for this unit so i'd be guessing... but my best guess might have something to do with power steering? just a guess though :o/ without a service manual or parts breakdown i have no idea what that is for honestly.