Is there a way to trick CS:GO into thinking I have another teammate for co-op missions?

You can't do it on your own and there is no way I know of tricking CS in to thinking you have a partner. Also, some of the missions require 2 players to get through the rounds, i.e. somewhere where you have to open gates with 2 switches that need to be activated at the same time.

If you solo queue or play casual games, just talk a bit and be friendly with your team. If you get a response from people you can request them as friends or straight up ask them if they want to do a co-op mission when the game is over.

Most of the people on my list are people that I've found friendly in matches I've played in. Also, with them playing in a match I played in, they are usually online at the same sort of time as me.

GLHF

I tried achieving this too, but it was not possible. You wont be able to start the search until the second slot is filled.

Anyways it would not be called Coop Mission when it is not supposed to be a coop Mission. It would be pretty hard for you to achieve the mission alone and end into not having fun playing the game.

You have to search someone who plays the missions with you.