THE ORIGIN OF ALL CIVILIZATIONS

Is it really reasonable to expect that during an invasion, all towns of a civilization would be torched? Usually, the capital might suffer such a fate, while the old leading elements of the society are destroyed, but if the conquerors intend to take over the society, they will not do the same for every city.

Well, not necessarily: put yourself in the position of the invaders: you burned down a capital and established yourself as the ruler of the country. Now what? Lifes goes on and you want to install yourself as a ruling class over the original inhabitants. You may move the capital to another prosperous city (Tyrinth), since Mycenae would cost too much to rebuild. The other cities prosper, but they're controlled by you and pay taxes to you now, so you're obviously going to let them go on.

Makes sense to me...

In ancient times, commercial power did not always equal military power. If the Dorians would have controlled Tirinth militarily, its citizens would have had no choice but to pay taxes. Besides, nothing says that they could not have formed a substantial part of Tirinth's population, or have made Tirinth their capital. Seems simple enough to me...

Anyway, you're right: we have veered off topic. Civilization: I would say that the prime requirement for it is agriculture. A surplus of food would generate a substantial class of people which can be dedicated full time to other endeavours, such as religious, commercial, administration, military and cultural  (arts, music, drama) matters. This also creates a non-egalitarian society. That to me is civilization. Some people get fixated on writing or architectural achievements, but I think that they are not really necessary, in order to term a culture a civilization. The first place in the wrold where civilization appeared is probably Mesopotamia. I think that we should avoid the term origin, since it implies that all subsequent civilizations spread from there.