For your first time backpacking, I want to congratulate you because you obviously did a lot of thinking about your itinerary.
However you're forgetting the time spent travelling from one city/country to another. Your whole program includes 35 nights exactly, leaving out the days or nights you'll be changing places. Like Skidrage said, it will take a looooooot of your time, you'll see.
Count one day for changing places, so you should stay minimum two nights everywhere, or you'll end up just a few hours free to visit. Ex : you leave Venice in the morning, arrive in Munich late afternoon, spend the night, visit Munich the next day, spend the night, and leave for Salzburg.
Overnight transport can help you save time, but it's exhausting. Good once in a while.
And if a you especially enjoy a country, why not take time to visit the countryside as well ? You'll see a whole different side of the culture. Worth it !