I'll respond beginning with the last question.

Hitler commanded the Wehrmacht to destroy every german house, kill every german civilian, whipe out everything that could even leave a clue of people ever living in germany.

Why? Well, mostly because in his mind, the german people let themselves, and most of all him down during the war. They didn't win, therefore they don't deserve to even exist. But also it was supposed to let the invaders down when trying to find anything useful.

It was one of the most perverted commands ever given during that war, at least to the regular Wehrmacht. Luckily though, most of them ignored that and surrendered.

Even after the war, there were the so-called "Werwölfe" (Werewolves), basically Hitler-Youth 'soldiers' who were taught in something similar to guerilla-warfare. But most of these terror-acts failed, seeing as those 'terrorists' were basically still children, that's not a big suprise.

The people left basically all the eastern areas, that's Pommern, too, for example, and as you said, mainly Prussia. Really, at least 60% of the refugees came from there, and that's because in Prussia the Red Army was exceptionally brutal, since it looked beautiful there, and they were quite mad to see how good a german worker lived.

Most of them tried to get to West-Germany, yes. Mainly because they heard that the british and american soldiers weren't as cruel as the Red Army was. And, eventhough they did commit a few crimes (random killing, theft, rape), those rumors were true, living in the west guaranteed you an easier life. (This is still true, btw.)

It started with the Wehrmacht moving backwards. As soon as they saw their own soldiers running, they got ready to run, too.

I'm sorry this answer isn't really organized, but I hope it answered some of your questions.