Your project sounds very interesting, you will have to tell us at the end how well it works.
Using PWM for precisely controlling the power is a good way. You can also do it with triac or the dimmer you did in that post.
It wont be difficult to add a feedback of the output power and to use it by the microcontroller to adjust the phase delay, that will solve the problem of mains fluctuation. I used to do in the past with analogue circuits. It was also a common practice to make regulated power supplies.
Mosfets will always be more reliable than triacs, but triacs are cheap.
I know that the chickens thread is different, I also remember you posting there, I was only joking about it.