Thanks Brucey for your thorough reply.
Particularly enlightening the figures in microns of cone movement. A 50th of a turn is just over one degree, and that translates into a very relevant tightening on tne cone! Impressive. That explains why the effect of a quick release on the adjustment is so massive. All hubs are too tight out of the factory, maybe they think it would look even worse if they sent out new hubs with a bit of play to allow for the quick release compression. Plus not everyone tightens the quick release the same. Difficult issue for the manufacturer.
I insert a set of steel washers on either side to make up for the dropouts and tighten the quick release to check the proper adjustment without having to mount the wheel on the frame. I get a more precise feel of the adjustment with the axel in my hands, But that is with regular hubs, dyno hubs are hopeless.
I agree that most likely my RHS cone is also damaged. I didn't take any of tne precautions you recommend to prevent the balls from dislodging. I oh ad already opened left cone and flipped the wheel around in all directions. I had read you before saying that sometimes you can move the right cone out a tiny bit, enough to squeeze some lube into tne bearings without undoing the connector. I tried hard to press on tne axel from the left side but it wouldn't move. I was trying to get at some of the grease on that side to get an indication of possible damage as the grease on the left bearing was like glitter, full of metal flakes. But in the case of the T785 at least, there is no movement at all. I should think that there is hardly any risk of the bearings dislodging because Shimano sell the whole internal assembly as a spare, ready to thread onto the shell and it must be constructed in a way that the bearing balls stay put, for obvious reasons.
Thus, I cannot understand how I could have a feel at the RHS bearing following your tip 3. It may not be possible with this hub. But it may be a a good idea to take out the whole internal assembly undoing the 32 mm cap nut. I would probably notice a badly failing bearing without the magnet drag at play, or at least get at some of the grease to check for "glitter". Is it a normal thread that loosens counterclockwise?
As I said, the axel would not move out even applying force on it, but it would wiggle around as far as the shell allows. No idea if that is normal with a healthy bearing, only way to know is having a fresh internal assembly to compare.
One Option would be buying two cones and attempting to replace both, assuming they are identical. The brittle wiring and the delicate "painted" insulation are a source of concern. If I fail I would have to get an internal assembly, so the entire innards would be new.
The cup race looked OK and I believe they have a hardening treatment far superior than the cones.