Alright, I've figured the problem out. One of the mods I've mentioned creates a 42kb overwrite of the "mpchristmas2017" folder which causes it to appear blank and means none of the cars from that mod can spawn ingame. After deleting the folder and its rpf from the "mods" folder, the cars appear as they should. The modified date on my computer's file explorer suggests it's NVR, but I don't want to pin the blame on any particular mod.

If anyone else sees a message claiming that one or more DLCs are missing - check the mods folder and see if there are any DLCs being overwritten.

By "up to that point", I mean Diamond Casino & Resort, as it's the most recent DLC. I didn't interrupt the installation process at any point; I spent 3 hours installing all 7 discs (don't know if a Blu-Ray drive would have done it quicker), and then another 30 minutes - an hour of letting it download 20GB worth of updates. I did have to free up an extra 3GB right at the end, but the installer paused for me and it allowed me to clear up space before continuing.

I'm aware that it's Online - I simply want to drive the cars from the DLC in single player such as the 190Z and Savestra as you can see from the Nebula, Dynasty and Hellfire in my later screenshots, but the car spawner can't find them even when looking up their spawn name and tells me the DLC is missing somehow, which means the cars are not in my copy of the game.

So, for whatever reason, the Doomsday Heist DLC is not on my copy of GTA V. I did a fresh install two days ago and allowed the launcher to download all the DLCs up to that point, or so I thought. This is a store-bought physical version of V (can supply photos for those doubting), and I have not made any major changes to the game as of yet, other than those highlighted below.

The version of V I'm running is 1.0.1737.0 according to the .exe properties, and the mod which alerted me to this problem was ikt's Add-On Vehicle Spawner. The only other mods I have installed at this point in time is Scripthook, NaturalVision Remastered and a modified popgroups.ymt to spawn DLC vehicles in traffic. I do not see how any of these mods could disable an entire DLC, especially one that came out quite a while ago.

Whenever I try to spawn a vehicle from the Doomsday Heist DLC, I'm greeted with the following message:
"This version of the game does not have the Doomsday Heist DLC content. Game version: VER_1_0_1737_0_NOSTEAM"

I have looked this message up but found only 10 results on google, none of which pertained to my problem. Here is an Imgur album showing how my game looks like, DLC cars spawning in traffic, and the odd absence of the Doomsday Heist. Is there any way to get the launcher to manually download a DLC? This is such a strange problem.

Thanks in advance for those who find a solution.