True or Myth?
Truth. Especially Santa Catarina.
Which Towns should i visit and how many days should i spend in which?
Stay in the South, don't come to Rio de Janeiro or São Paulo, stay bellow them. South of Minas Gerais can be comfy af too. I don't know which cities you can go, man, do a google search and you will find something interesting with the tips I gave you.
Any Nature Parks, Hiking, National Parks, Horse Riding Paths & Areas down there?
Yes, there are farms (fazendas) were you can visit, play with the animals and eat there. Eat the brazilian desserts, the ones that come in a mason jar, they are pretty good.
Are Brazilians in those historical German Areas still behaviour-wise german?
I would say yes, some of them even speak german.
Or do they try to rip me off when selling me stuff every second?
Nope. The stuff you buy may be overpriced for everyone.
Do Brazilians care if i am respectful by at least trying to speak Brazilian (asking stuff, greeting etc.) to them?
Yes. One time I meet a german, he stayed here for a week and the motherfucker was speaking almost perfect portuguese, I was really impressed, and complimented him for that. I thought he was brazilian even, at first I talked to him like he was a fellow countryman. So, I would say, try to learn portuguese now at Duolinguo or something, and really try to connect with people, they will appreciate it.
Any Brazilian customs in those Southern/German Areas i need to be aware of?
Not that I know of. Just be yourself.
Any People targeting Tourists in the South? (Prostitutes/Girls stealing, Drugs in Drinks, Fake-Cabs at the Airport like in New York where they aren't official cabs and then charge you 5x the price?
Don't know anything about that, but Taxi drivers around here are not always realiable, but don't let that discourage you, most of them are honest, but I do recommend you to take a Uber or a 99taxi (a brazilian uber) every time you can.