So this is a reply just in case someone find it useful one day, given this topic didn't exactly get much traction...
So eventually managed to get indi_v4l2_ccd working again with the SPC900NC modified for long exposure controlled by LED.

In addition to the modification documented here , I had to also patch the indi_v4l2_ccd in order to avoid reaching a part of the code which - according to the comment should anyway not be reached - where the capture was failing.
The patch is attached bellow, but BEWARE: I'M NO INDI DEVELOPER AND I DONT COMMIT TO ANYTHING, DON'T BLAME IT ON ME if it breaks your device, insult your mother or reelect Georges Bush (<- a geek reference is hidden here!)

Thanks to @geehalel for his help and for updating his driver for recent kernels (5.4)

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