It isn't hard to understand your consternation over sending your Mavic Pro away to DJI with only an apparent gimbal fault, but getting back from them an aircraft that won't fly at all.
I wonder if I can be a little bit of a devil's advocate here, suggesting the WD40 may well be the cause of a problem which progressed to the point where there is little if any functionality now.
WD40 probably has a well deserved, world-wide reputation as a lubricant. But I am picking that in your situation, it was almost certainly the wrong product for use on the Mavic Pro's gimbal. I don't know what the chemical composition of WD40 is, but it is likely to be corrosive to electronics.
The point you have made about the Mavic being "perfectly" functional, other than the gimbal problem, immediately before you sent it away, could well fit with a situation where WD40 has taken time to, firstly, come in contact with an ECB - flying at speed for a long distance may well have helped that process along. Once there is contact with an ECB, then penetration time is a matter of how corrosive the chemicals are in WD40 to the Mavic Pro electronic components.
Once penetration had happened - even with just the smallest amount of the lubricant - the sort of selective effect you described happening to your Mavic Pro's motor management and avionics is inevitable, even if it does take time.
I'm not going to judge the service you have had from DJI service agent, beyond suggesting they probably didn't do a very good job in detailing their rationale for suggesting your Mavic Pro was a write-off. However, I do suggest, that however well meaning your intention was to use the WD40 around the gimbal components, that was quite possibly the beginning point of the Mavic Pro's eventual failure.
A good point for all UAV operators to remember, is NOT to use such lubricant products anywhere on a UAV. Be aware, lubricant does not disperse dust or grit, it will merely provide a barrier between the UAV's moving parts and the foreign material, in some situations turning into a form of slurry with even worse ECB degrading properties. At the very least, use good quality, electronics grade, compressed air - in this case, all round the gimbal and its housing, making sure air vents are also cleared of foreign material. At the most, grounding of the UAV should be considered as a priority, with the UAV being stripped down to its basic airframe to be thoroughly cleaned. Once considered airworthy, testing should be planned for incrementally longer, faster, higher, further flights until absolutely satisfied there are no issues with flight control management systems and avionics.
If that all sounds a bit OTT. It's meant to be! This forum is full of "sad" stories about otherwise careful flyers losing their pride and joy; following what might have been seen as a previous "minor" event. While our UAVs are obviously much, much smaller than their larger cousins, they are still aircraft. There are no lay-bys up in the sky - you can't just pull over and shut one down when something goes wrong - so, inevitably they will either drop out of the sky, or perform some bizarre behaviour like flying off, or, if you're
lucky , they can't get airborne in the first place. Carry out pre-flight checks, including batteries and flight controller, every time. Keep a log and schedule a maintenance plan for each 25hr block of flying time.
Most of us wouldn't even consider recreational flying in a light aircraft that didn't have a good, written, service record, never mind taking a flight on board a commercial aircraft without the knowledge of there being a similar servicing record!
Happy and safe flying. ?