While I can't say that I really got to experience any composite mode on a composite display back then (although a built-in monochrome monitor accepting an NTSC signal was used)...
Very good job over there! I can't say that I understand everything, and I'm sure you know what's done here better than me 😀
There's one goal I can agree on: Try to make things as authenticate as possible. Not smooth, nor pretty; Just authenticate.
However, I realize that goal may require the ability to adjust some settings like hue and brightness (please correct me if I'm wrong). Thinking of the way it's done on older TVs and computer displays... Well, if DOSBox had a GUI with knobs or something, these could be used to control the settings, probably; And then, of course, the settings would saved to somewhere. (I *think* you wouldn't expect a TV to reset it's brightness, right? 😀) Maybe the current conf file can be used for this (already discussed, I know), and maybe a different file (probably a bad idea).
I guess the knobs are one of the few examples, where a touch-screen would make the controlling of these more similar to the real thing than a mouse would. (Hehe)
Well, think that I'd be done now. As usual, there's zero promise that any suggestion is to be incorporated into an official DOSBox release.
Good luck to all!