I did this in my miner with an optocoupler and a power transistor (2955 but that is overkill). PWM controls the cheapo fans just fine, I'm only use 2 pins per fan, relying upon temperature to set the PWM rate with no feedback.
Yup. If the code detects no Tach output it will just run open-loop and set a PWM signal for the fan. So 2 wires will work. I noticed that these cheap fans don't get much oomph below about 75%, and stall around 25%. I'd say only the top half is useful, but they're quiet up to about 80-905.