Reddit mentions of RioRand RR-PWM-15V Low Voltage Dc 1.8v 3v 5v 6v 12v 2a Motor Speed Controller Pwm,

A 12V Power Supply

Simple PC Fan, 12V

PWM Controller

The wall wart will supply 12V with a max of 1A of current. 12V computer fans of that size use at most 0.5A. The PWM controller will control the fan speed. It works by sending out DC pulses, also known as a square wave, of different lengths based on where the knob is set.

To hook up:
There are 4 terminals on the PWM Controller. Look on the bottom to see the polarity. Should be +,- Power and +,- Motor. They're screw terminals so she won't need to solder anything.

Simply hook up the red fan wire to the Motor + terminal, and the black fan wire to the Motor - terminal.

Next, WHILE THE WALL WART IS UNPLUGGED, splice the cable so you have some copper showing at an end. If it has red and black, simply repeat what you did with the fan but use the Power terminals. If the wires are the same color, you can use a multimeter to figure out the polarity.

Schematic

No major strain on the fan, it just lowers the voltage, not perfect but probably not going to set a fire pulling .3a. I use them on my RC cars for LEDs and stuff. This is the one I have used many times on different projects. It takes 12v and lowers it to either 6v or 5v, up to 3a. I had it at 6v first, but I could feel blowback so lowered it to 5v. Works perfect and is whisper quiet.

No cant be used as a speed controller. It's constant output. If you want to adjust speed, you want a PWM like this. That will allow you to adjust the speed. The good thing about PWMs are they dont lower voltage, they lower the pulse width.

Yeah, I basically want to have something that can range from 0-100% (or close to it) duty cycle.

This one here says it'll go from 0-100%. And this one says it'll do ~1-100% duty cycle.

Is that a lie? Will it be more like 98% or something?

Attempting to use a resistors to reduce the speed of a DC motor is ineffective. Specifically, it will not stretch the battery life.

The correct way to reduce its speed is by varying the voltage. Use a DC motor controller.

For example: https://www.amazon.com/RioRand-RR-PWM-15V-Voltage-Motor-Controller/dp/B00N30UK2M

I'll warn you that even though I bought buttons from the same manufacturer in the same style, he buttons were extremely different brightness. If that happens for you, I used a rheostat to turn down the power to the light.

this is the one i used: https://www.amazon.com/gp/product/B00N30UK2M/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

Yes the fan A header on the board is software controlled and not really suitable for being used to cool the v6 unless you leave it at always 100%. Wire it to your part fan and you will be fine. Alternatively, if you're lucky and your fan B header works and you can just plug into that instead. I shorted out my fan A header and had to go with wiring in two of these for my cooling needs: https://www.amazon.com/gp/product/B00N30UK2M