Hi, Hidri. I am not entirely sure which questions or issues you were asking or addressing, so I apologize if my responses are off the mark.
1) My understanding is that 180 degrees for 30 minutes will destroy the lactic flora, regardless of whether the microwave is used or not -- yes?
2) In my first effort, I brought the milk up to 180 degrees, stirring and checking every minute until it got within 20 degrees, and then stirring and checking every 30 seconds. In the second effort, I brought it up to temperature, and then ran the microwave at 20% for 5 minutes at a time, stirring and checking each time. At this power setting, the temperature remained pretty constant.Obviously, there will be localized heating of the milk when the microwaves are acting on it ... I'm not sure how different this may be from the localized heating at the surface of the pan when heating on the stove with an element that cycles on and off.
3) Right, but as I understand it, the point of the heating and holding at heat is not just to kill germs; it is also to cause a change in the protein structure, resulting in a firmer set. I suspect there is also a bit of concentration of the milk happening, as some water is lost over the 30 minutes.
4) Using whole milk and holding at 180 degrees for 30 minutes, the firmness of the yogurt is remarkable, so I have not felt the need to add powdered milk.
As I reported in my second post, the second "microwave" batch that I made turned out very well indeed -- maybe just a bit less smooth in texture; I'm not entirely sure about that -- but very firmly set, and delicious. If I can make my "Rube Goldberg" stirring device, I think it will help to eliminate overly hot spots, but even without it, I am happy with this procedure, particularly in terms of its convenience.