1. 3 days is barely enough for London if you don’t do a day trip to Stonehenge. If day 1 is arrival day from North America, you probably need another day.
2. Normandy is only a reasonable day trip from Paris if you take a tour and you still miss a lot On your own, an overnight trip is better. Mont St. michael can be done as a day trip if you want to spend 8 hours of that day on bus and train or slightly longer going by car. You could hit both on a VERY rushed overnight trip by takind the high speed train to Rennes, renting a car and returning it in Caen and train back to Paris— quite possibly a budget buster and you probably won’t be able to hit all the places you want to see in Normandy.
3. Stay in just one place on the Riviera. Nice is central, Cannes and Monaco are both minutes away by train, so easy day trips.

I would generally recommend train for this trip, with a rental car or two if your budget allows a splurge. Your budget seems OK. I’d figure $1,200 each for airfare (assuming an August departure date)$150 each per week for city to city transportation by train, bus and cheap flight ($250 EACH per week by rental car) and a minimum of $75 each per day for hostels, food from street vendors and markets, admission fees, local transportation by subway, bus and a lot of walking, day trips, laundry, misc. $100 each per day would allow some splurges like a rental car for a day or two, some pub and cafe meals, more than the minimal amount of wine a beer covered in the basic budget, and the extra you’ll need for your very expensive stop at Oktoberfest. $150 each per day would upgrade to “moderate tourist class” hotels, more cafe and pub meals plus additional splurges like staying in central Munich, Oktoberfest beer tent reservations and a couple of meals in nicer restaurants.

Why are you backtracking to London? Look for a multi-city flight, into London, out of Munich or Frankfurt to avoid the time and expense of backtracking to London. “Open jaw” flights like this are often only a little more expensive than a roundtrip —probably a savings over a flight from Germany to London during Oktoberfest.

Agree with Oldlady’s advice. I would allow more time for London, esp since you will be jet-lagged. A day trip to Stonehenge is feasible but (in my opinion) rea-e-ally pushing it. Doing a day trip from Paris to Normandy is pretty much impossible.

About the beach visit out of Rome, you could go to Ostia Lido but it’s probably not worthwhile.
You could also take the train to the Amalfi coast, which is gorgeous, but you would probably want 2 days there, not sure you want to do that when your next destination is Venice, so far north.

I agree with Oldlady that flying out of Frankfurt or Munich would be much more convenient and probably not that much more money.

I would tend to agree with Cil & oldlady on most of their recommendations… Having lived in Rome, Copenhagen, Madrid for a 10+ years, I’ve done all those trips… but not consecutively. One alternative to trains might be looking at low budget Euro airlines. Easyjet now hs a US website. We’ve flown into Prague often on Czech Air, and Munich is a great departure point – and has the best-rated duty-free shops in Europe! So open jaw and low-cost carriers might save you some time … and maybe even some money.

I would go on the bahn site and look at the time it takes to get from one location to another, like Venice to Salzburg. You may find you need to delete a few locations, or as you suggest, increase your number of days!

In Italy, you could easily spend a week in Rome. I lived there 3 years and still have 50+ sites I want to see. You could also check out Sperlonga for a beach destination – quite accessible by train and certainly more attractive than Ostia, Fregene, or the Lazio beaches. I’d add more time to Venice and might even cut out Salzburg (been there a few times – it was “okay” to me)… and a little out of the way. We often opted to fly into Vienna, a city we just love.

I’m sure you’ll get many more suggestions. Just give yourself enough time to “experience” your travels and not just “see the sites.” Good Luck!