It was long due to make this.

What you need to know: once you use the optional "read only" option none of the in-game modifications will work anymore. Toggling mods on or off, along with adding and removing downloaded mods has to be done entirely manually in the text file, otherwise you run the risk of having the load order shuffled every time you load the game.

Is it inconvenient?
Yes.
Can I make a mod to allow you to change the load order in the game?
No, that is not possible, Sorry.

One thing that I forgot to mention in the video, the "1" at the end of a line is a boolean.
"0" means the mod is disabled, "1" means it is enabled.

1. Open "Steam\SteamApps\common\left 4 dead 2\left4dead2\addonlist.txt"
   
2. Find the mod you want to have higher priority and move it over the one you want to have lower priority.
   
3. Save
   
4. Enable Read-Only by right clicking the file, open "properties" and hit the "read only" checkbox.
   Enabling "Read-Only" is optional. Not enabling it will cause the entire list to shuffle when you have a lot of workshop mods installed. It does not seem to shuffle when you don't have any workshop mods.

If you do enable "Read-Only" then you will also have to enable or disable addons manually, by turning the "1" to a "0" to turn it off, and vice versa.
You will also have to manually add addons to the list when you download workshop mods, which is done like this:

1. Open the workshop folder "Steam\SteamApps\common\left 4 dead 2\left4dead2\addons\workshop"
   
2. Change the order to "by date" so all new mods are at the top, copy the name of the thumbnails of your new mods and paste them into your addonlist.txt, along with the folder structure like all other mods.

You would also have to remove them manually, but having a name in the list that is not actually used will not cause any errors.