A few thoughts:
I worked for a time in prepress. A dark, moody image that looks good on a computer display, may not translate well to print. High quality printing on glossy stock may be OK, but cheaper quality magazine/newsprint can be problematic. It's one of the difficulties delivering images to clients if they don't know or can't tell you specific end uses. In any case, nowadays with most things being displayed on an electronic device, you probably can get away with things on the dark side.
Regarding the cameras you asked about: I only have extensive experience with the M240 and would say that while 6400 can be usable, I prefer to keep 3200 as the max, and if possible, 1600. At 3200, at least with my camera, banding starts to become evident and the files are not all that malleable in post. You need to be pretty spot on with your exposures. Also, if you're someone who likes to grab a couple quick shots to ensure you have one with everyone's eyes open, a good expression, no camera shake or subject motion (if at marginal shutter speeds), as soon as you set the M240's ISO above 1000, it goes from ~3fps to ~1fps. This really annoys me, but might not be a problem for anyone content with shooting in single shot mode.
Regarding the a7S vs. a7II: I shot a few weeks with the a7S. It's strength at high ISOs is good color quality, however, I wouldn't consider it to be all that much cleaner (also based on comparison with my 1DX files) for luminance noise. The primary reason I would get it over other a7 cameras is the fully electronic shutter, which is dead silent. However, if you're using native glass on it, you can hear the aperture stopping down (generally very minor and probably irrelevant in dark situations) and focus drive, particularly if adapting older screw-drive alpha lenses. With adapted fully manual glass, of course it's totally silent. The a7II gives you IBIS, which can be very useful at marginal shutter speeds with normal to short telephoto lenses in this kind of work. Usually people aren't moving around too much, so image stabilization will buy you some sharpness. Another consideration is the size of files you're delivering to your clients. Is 12MP enough? For event type work, I think it would be. In fact, past experience has been that the average non-techy person's computer system tends to bog down with huge files. You could very well downsample 24MP a7ii files to ~12MP and it will mask the noise difference between the two. Downsampling with a touch of sharpening will give you a sharper 12MP image from a higher resolution sensor than from a native 12MP sensor.
Something else to consider with really low light work is how well EVF based systems will or won't handle such conditions. Will the EVF become really laggy and grainy? Will AF hunt a lot more? A concern I would have is how unusable a camera might become if the AF system starts having problems focusing. You could completely miss moments waiting for something to lock on.
IMO, the fastest focusing system for really low light is often a rangefinder, followed by some of the higher end DSLRs with -2, -3EV AF systems.
Another M mount lens to keep in mind if you're considering the a7/a7S route is the 35/1.2 Nokton. It will give you the moderate wide you're maybe looking for, with better technical performance than the 35/1.4. It also is one of the few RF wides that works really well with unmodified a7 series bodies.
