Thank you guys for your creative suggestions and compassion
I think the best short-termed solution would be pressing the lower C with LH, which Chris for the first time suggested and Monica and Scott assented but I wanted to reserve as the last resort. (Eddy's and Alexander's suggestion find I also well-functioning.) Frankly speaking, I thought it would be a sort of cheating, but as the small size of my RH (btw my LH reaches 9ths, it is a bit larger than my RH) is officially approved now
, I may throw that feeling away. Besides, it didn't work well before I opened this thread, because my digital piano is so sensitively responsive that my "cheating" was hearable. But my facility was much improved in the meantime and I can perform that well also on my digital piano (but I won't use my digital piano for a possible recording, of course).
In your case, I would prescribe (Rx) a lot of work (scales) with 9ths and full (5-note) fully-diminshed 7th chords (I. Phillip's Exercises for the Independence of the Fingers). Both of these will "open" your hand to what ever extent is maxibally possible. I feel for you.
I would try your prescription willingly, Eddy. But where can I get the score? At least not on IMSLP. Or could you suggest a good printed edition? I know you mentioned these etudes several times and would like to give a look at it.
Now I see why block chords give you trouble, you can only just reach a bare octave, and anything in between multiples the difficulty. You'll probably have to leave some notes out to get it manageable.
press other chords on the snipped score. C-E flat-C is difficult to press, but for example C-E-C is easy. Maybe the short pinky is the reason.