You can't, they clearly are all independent mods, that's why they show up independently. Maybe if some of the Porsches would be part of one and the same mod you could chose not to show custom configs somewhere in the options menu, but I doubt any of them are not separate mods. If this disturbs you, you can always disable mods you don't want to use by choosing the mods tab in the UI or the mods menu in the main screen. This works immediately without any problems.
Yes, however they're packed as separate vehicles. If you go into the zip and double click on vehicles, you'll see many different folders. There is no current way of sorting them easily.
You don't. The screenshot you posted implies that you downloaded AgentMooshroom5's Old Private Mod Pack from his YouTube video, composed of mesh-slaps made by various users (myself included). It's worth mentioning that the vehicles were originally separate from each other, but due to an incident that I would prefer not discussing in greater detail, AgentMooshroom5 made the decision to bundle everything into one .zip file before publicly sharing the mods. (Similarly, some mods on the repository, such as WBIMP, contain a wide variety of parts for many official content vehicles and said parts are thus also packed into one .zip file.) Which leads me to the following point - the game treats the entire mod pack as one mod, meaning if you disable it in-game, all of the vehicles in that one pack won't show up in the vehicle selection menu unless you reenable it. But even if you were to have just the mod pack enabled and the game believes only one mod was loaded, that doesn't mean the performance won't be affected; in fact, the more vehicles that show up in the vehicle selection menu from the one mod pack alone, the longer it takes to load a level or switch vehicles (which is akin to having a similar quantity of separate mods enabled).
