Usually using two adapters results in the image circle being too far away from the film plane. Even if back focus is still correct, you'll get sever vignetting.
you mean the adapter vignettes because the c thread part is too deep compared to the rear element? The back focus will stay the same unless there is something wrong with the adapters.
Efs lenses are not very well suited for film camera use, too much ramping, focus problems and only electronic aperture adjustment. I suggest using old manual stills primes instead for tele, like nikon ai-s or pentax super takumar/smc takumar lenses. If you choose m42 lenses you can fine tune the adapter by yourself with sandpaper if the ffd is not right. For wide angles you can use certain cctv/machine vision lenses like those Chinese 8mm c-mount lenses found on ebay. If you need a zoom you can look for old c-mount tv zooms or alternatively a b4 zoom with ef adapter and attaching the combo to the camera with a ef to c adapter. Good b4 zooms are expensive though and you may need to use the extender to avoid vignetting.
Still lenses usually don't have accurate focus scales anyway so it is really only up to whether it can be focused to infinity or not