So many question to answer first one is really complicated and this can be best answered by
skacikpl or
erxv because they have experiance in adding new things fx, objekts and soo on in game, i think you will have to find file for that quest and add it, this knowledge can improve some quest sceen apart from that first that comes to my mind is with chrones and when they first appear adding fog should make this scene much better.
White smoke can become dark at night you just need to change it inside mcolorgroups i am not certain but i think fog is for both fog, semi volumetic clouds used in KM, and with mod in Skellige, but maybe it is also for smoke. Also smoke is very effected by global light sun color. I am @ work now so i can't test this.
Also about geralt specific lighting, in both cutscenes and out off them, one value that have neer as much impact like camera parm, is inside global light, and i think it is called ambient color base or something like this, i will eddit this with right answer but belive me it effect Geralt soo much, like for example you can see this in effekt in Kaer Morhen. Go in are with shadow, and zoom on his face and beard, it will have green tone, and this can be sean on both beard and hair. What i want to say to you is this, it is realy hard to make Geralt apper 1-1 in very scene for every interior cave and soo on. They way you take with script in my opinion is wrong not just because of BAW region but in general, you just lose a lot of control over many instances. You want to make mod as realistic and i salute that but do know you will lose a lot of details and that will impact on general feal of the game. Best aproch is to use as litle in your cutscene lighting, i use only two parms in my new version of the mod. You can try to mimic this, also feal free to use my files, and see how you can incorporate them in your mod.
I will also post soon new version of Vanilla enhanced, with new fixes, also if you want feal free to use this also.
Learn from my files. Because in this new version i did fixed all transitions for weathers and soo on and feal free to use my interior and cave files from Vanilla enhanced because they match yours.
Also one thing about water, it is also effected big time by active weather so it will not look same for both nice and bad weather. And value you inputed can be achived with diferent values but with diferent lue parm tab - W
You can input realy bright color but if you lower it brightenss value it will drop down and when you for example screenshoot it and extract color with tool you will see diferent value that you input in ENVs only bacause of lue - W value.
My god now i hope someone will understand all the shit i wrothe here because my English is bad.
Also i hope it will help