What are the risks of powering fan AC motor with Triac dimmer?
The speed control may not work very well. Nearly all AC fan motors are induction motors. Induction motors are inherently designed to operate at a speed determined by frequency with very little controllability by voltage reduction. However, low-powered fan motors of the shaded pole and permanent split capacitor (PSC) type of induction motor can be controlled that way. Your 300 watt motors are a little larger than the usual motor (150 W or less) that is controlled that way and may overheat if they are not designed for that duty. They may also have a more limited range of control.
Electronic voltage reduction distorts the waveform of the current drawn from the source.
I'm going to replace current resistor potentiometer with arduino-controlled triac scheme, probably this module.
It appears that you will need to program phase-back firing control for each fan. Have you considered that?
How bad the EMI gonna be (would I be able to plug audio amlplifier to the same line)?
You will need to design filtering to deal with the EMI
Would it substantially influence reactive power section of my electricity bill?
A distorted load current has an effect similar to reactive current and will reduce the total power factor, but not the displacement power factor. It is likely that the effect on the meter will be to increase the reactive section of the bill. The percentage increase may be small depending on other loads. The addition to the reactive power is not arithmetic, but proportional to the square root of the sum of the squares of the distortion current and the conventional reactive current.