Thanks Christophe, something we could do is return the number of valid manifold vertices instead of a success bool flag: This would allow us to compute everything we need while not having to change the signature that much? What do you think?

I do not think it would be that much better: for one, we do have to test for success and maximum recursion depths within mnee.h (so we already compute them internally); and for second, the function is only used once, from shade_surface.h, so its signature (slightly expended here) does not matter much.
This being said, if for some obscure reason it is preferable here, in a mega-kernel, to have less return variables, then I obviously would be fine with changing this code.