Walk assist working means controller is ok, being slightly warm should be normal as slow speed draws higher current which adds heat.
It is likely the bike has been knocked or fallen over and either the PAS or a brake sensor is at fault.
Your controller connections are typical of waterproof type, Black one is battery. No data cable which I assumed as they are fairly common with Lishui.
Cable tied to the battery wire is motor connector (5 pins), mid size connector with Black inner (8 pins ) is the main trunk cable connector (this supplies power to display, brake sensors, lights and throttle etc,etc not all may be used ) and small connector yellow inner (3 pins) is PAS.
Testing is tricky unless one knows how to, I doubt if the shop new what, how, where or why to test.
Connect everything up again.
Two areas I would look are:
1. PAS set up on the bottom bracket, it should be a simple sensor and magnet disc. (Show us a pic).
Ensure the gap is a bout 1mm.
Is there a Red led on the sensor ? If so in a darker place check that it blinks on/off as you rotate the cranks (make sure the bike is off the ground or up side down).
2. Check brake levers do they return to the fully off position, sensor/s fitted may be at fault and one is faulty. Trace the brake sensors to the waterproof 1t4 /1t3 connection near the head tube disconnect both, then try for bike operation.
If the bike works connection each brake in turn to find the one causing the fault.