The panic comes from trying to fit in too much. The most common mistake in travel is jumping from city to city every day or every other day, and spending too much time packing, unpacking, and traveling from place to place, instead of enjoying things. The words "stress/panic" and "vacation" should not be in the same paragraph.

My suggestion is if you want to visit Germany/Austria, then see Bonn, Salzburg, and Vienna. If you want to see Italy, forget Salzburg. Salzburg is fabulous, but you just can't do it all.

Salzburg, Venice, Tuscany, Florence, Rome, in 8+ days is no vacation. It is madness.

Consider flying from Bonn to Venice. I agree with your daughter's friends, the train ride from Bonn is too long. It would kill a whole day. After a few days in Venice hop on a train to Rome, and spend the rest of your time there. With only eight days, Venice and Rome is about all that you can do. And, you will be at each place long enough to get a feel for them, with time to wander the back streets, not just rushing in and out of hotels.

I would panic trying to squeeze Salzburg, Venice, Florence/Tuscany, and Rome into 8+ days. Venice and Rome are spectacular enough to warrant your 8 days. You can always throw in a day trip if you want, such as a 1.5 day stop over in Florence on the way to Rome.

Anything more in just 8 days is not advisable. You will be skimming the surface of everything, but not really seeing or getting to know any place.