Pressed for time, so just a few observations for now:
Using companion jpgs after the fact to try to learn about raw file exposure brings the same problems as trying to use the jpg-driven live histogram in real time to set optimal exposures for raw capture. Camera-makers’ default jpg settings are generally contrasty and highly saturated. If you use default jpgs to judge the maximum non-clipping exposure you will generally underexpose, often by a substantial amount, relative to what would have been optimal for the raw file.
In principle you can adjust the jpg parameters to make the jpg a somewhat less inaccurate guide to what the raw file needs. Many raw shooters do that with a one-size-for-all-situations compromise, turning down the contrast and saturation. A very few (not including me!) who are willing to fuss a lot more go the UniWB route to get a much closer match across a wider range of scene conditions.
Unfortunately, mirrorless cameras, even as they provide more convenient access to the live histogram, make these approaches more difficult. Because jpg parameters also control the live view rendering, extreme settings generate a view that is at best unpleasant and at worst impossible to use. An OVF in a DSLR or rangefinder camera allows the use of extreme jpg settings without disrupting the photographer’s view of the subject, but of course you lose the live histogram. The exception among mirrorless cameras would be the Fuji X100 and X-Pro series, which can be set to display a histogram in the viewfinder even in OVF mode.