Depending what came with that DLC, you may or may not be missing some peds or spawn items/props as well iirc.

Not sure what you mean since none of the mirrors on the cars actually work other than a very basic reflection that may as well be a silver sticker.

Aside from seperate maps that load in when you enter a location, the single player map is pretty on par with the online version, and as suggested, a trainer can load in the other maps, just not always with full functionality.

VisualV/NVR for sure. I would suggest QuantV except that it's nolonger available as it was also pretty good in that list.

Always the latest version, except I don't let it auto-optimize my gaming experience as that tends to mess everything up. I know how to tweak and don't need some cobbled together algorithm doing it for me.

I don't use any third party launchers. I just swap the GTA folders to the one I want and press 'Play' in Steam and it loads in the R* launcher (that no one asked for) and then loads the game just fine, including my modded SP one.

Do you run it and LSPDFR at the same time? Also in order to have LSPDFR work, you need RPH, which could be the conflict. Typically I disable Open Interiors when running LSPDFR since I won't need its features while on the job.

@TR71777 It initially came across as "this mod won't work because it's not been updated and it needs updated".
If it works for everyone else except you, then it comes down to troubleshooting your mods to find the conflict and it's not a fault of the mod itself.

Sure. Let me look into my crystal ball to find all of the information missing from this to then give you a solution.

I'll second @Reacon in that it works as intended and only with what it was coded for. Besides the vast majority if the new interiors are their own map spawns now vs being in the main map, and so trainers or specific mods would spawn that map for you.