If you'll notice on the mod thread, it says A. The most recent version of Volumetric Effects is 1.2, not 1.0 and B. Even 1.2 is only "COMPATIBLE WITH SoaSE 1.18/Entrenchment 1.41". You must always check that your mods are up to date for the given version you have. Sadly, there is no graphics mod that is up to date for the original Sins.

That said, if you get the 1.2 version there is a chance that it might work with 1.191. Further, if it doesn't you should be able to just delete the GameInfo folder and get parts of the mod to work correctly.

Yeah you have the process right, just need to find a mod that works. For regular Sins you really don't have a lot of options, but Maelstrom is one of the better ones. SOA2 and Hammer Real Sins (Volumetric Effects built in) also look good (I haven't tried them though). While it doesn't always say what version they are for, this is one of the best places to find mods, besides this modding forum of course.

No, but just because it has an Entrenchment/Diplomacy sticker doesn't mean they don't have a Vanilla Sins version avaliable (like Maelstrom). It usually has a link to the site of the mod itself, so just click on that and check there if a Vanilla version.

That said, most mods are for the expansions only as A. Its a pain to have to update several different versions of the mod and B. The expansions are much easier to mod, and you can do more things with them. Thus modders prefer to work with them by far.