Welcome to the forum.
The sparking with the power off is the capacitors discharging and is nothing to worry about.
If the wire gets red hot but you can't make it arc then one or more of the rectifier diodes could be blown. You need to isolate these and test them individually with a resistance meter. If you find a duff diode replace them all.
The motor drive is sometimes taken off the DC welding voltage, we need the wiring diagram to verify this. This is done so that the motor speed is set roughly as you set the welding power, the speed knob is then used to fine tune the wire speed.
I had a similar problem on my snapon 180c .
The trigger relay was clicking but no wire feed I also chased all around the circuit board ,it turned out to be the trigger relay although it was clicking the contacts weren't quite making a good contact, metered it with an avo 8 not a digital to find fault.
Changed the relay and all was working again , the relay was glued onto the board had to strip out and fly lead onto new relay.
Hopefully this might help
Been following this one hoping you'd get it going again. When you suspended the PCB and it started working you could have wiggled a wire or component and made it briefly work. Perhaps trace the wiring and check / replace anything untoward? Could be a finicky relay as arceyered says above.
Ahhhhhh
You most probable have a dry solder joint !
As the wire feed control is mounted to the pcb and then mounts to the machine that would be one place to reflow the solder.
Then reflow the solder on all the plug pcb pins (6 pin plug and the spade connectors).
Then due to it's weight the pins of the on board transformer.
And finally the relay pins.
To reflow you need a nice clean and HOT soldering iron and if you don't have a solder sucker then just add some fresh solder (take care not to over do it and short a track to another.