It sounds like your flights are already booked. Where are you flying home from and do you plan to spend any time in Europe after the end of the tour?
I'm always leery of connecting flights right after a long-haul flight - there are so many ways you can be delayed: inbound flight can be late for any of several reasons, you can get hung up in a long line for passports or luggage, it can take a long time to check in for your next flight, get through security, and so on. In your case, it sounds like you'd be on a separate ticket which probably means, if you miss the flight you're out the money and booking a new flight can mean more delay and much more $$$.
You can take the train from CDG to Strasbourg (there's at least 1 direct train a day, and lots with 1 connection). You'll save money by booking as early as possible, but you either have to leave lots of time to allow for delays or take a chance on missing the train. Or book a full price ticket so you can exchange it if you're late. Or just spend a night somewhere and take a morning train. Maybe stay at an airport hotel and spend the day in Paris.
I like the idea of Strasbourg and Colmar for 3-4N. Then maybe train to Munich 1-2N, train to Salzburg at least 1N before the tour. There's plenty to see in Salzburg for a full day before the tour.