With that budget, you won't have to "live like a hobo" but as other suggested, you do have to tell us what your interests are, do you want to go out drinking with friends or would you rather see the monuments and go to bed early, what you would accept as you accommodation, whether you'd like to eat at fast food joints or try country specific cuisine.
And without the itinerary, we can't help since different countries have different standards and prices.
That France and Switzerland family stay period will let you keep a lot of your money but then again, you would spend a lot of money in Scandinavian countries if you decided to go there.
Did you consider Baltic countries, Balkan countries or even Turkey?
Balkan countries are, for example, a lot cheaper than the Western Europe, especially Montenegro, Bosnia, Romania and Bulgaria. Even Serbia is quite cheap. I recommend homestays in Bosnia (very cheap, especially if you're not alone) and if you want to live like a king (or prince, as you say), go to Montenegro and stay in fancy hotels and enjoy the culture.
If we put France and Switzerland to the side, Spain, Portugal, Italy, Benelux are quite expensive but if you stay in hostels (and there are quite a few cheap cozy ones) and make bookings early enough, you should even have some spare money. Museum and monument tickets are expensive in all of the Europe so watch out for those. My opinion is, if you don't like something like modern art, don't visit modern art museums. Save that money and use it on something you enjoy more.
Yeah, traveling by train is your best option as it is cheap and fun but I wouldn't suggest an Interrail pass if you stay at certain countries for a long time since it's too expensive then. Use country specific passes if you want to travel a lot inside a specific country and if you want to go from one country to another, check the price for a single ticket(s) and compare it to the price of Interrail passes (either 175 euros for a
5 days travel within 10 days or 257 euros for a
10 days travel within 22 days) and then choose a better option.
France - no accommodation costs and you'll eat at home a lot but you will most likely go out in the evenings and you'll be visiting museums and so on so you would probably need 50 to 60 euros a day, although, if you just take evening walks through the town, go for a drink or two, you could spend as low as 15 euros, especially if you take photographs. Being a photographer really helps your budget because you don't take track of the time and it's bed time sooner than you'd think.
Monte Carlo - I presume you will be only passing through it so count on spending maybe 30 euros there for a meal, a drink and maybe some souvenirs.
Italy - I don't particularly like North Italy, it's crowded, people aren't friendly and are always trying to scam you and everything is way more expensive for the foreigners. 70 to 80 euros a day if you stay in a hostel that isn't in the strict center of the cities. For things like ice cream, drinks or cheap meals, ask the locals if they seem likely to help. Take photos of famous sights but aim for parts of the towns that aren't visible to everyone, there's a bunch of small alleys for you to explore.
Slovenia - Not that expensive but you'll be probably only passing through since it's small. Check out Ljubljana, Maribor or Bled. If you're not staying for the night - 30 euros, if you are - 45-55.
Croatia - Also, not as expensive as western Europe. Hostels are 15-25 euros a night, eating for a day + one museum a day - 20-25 euros. Here's a patriot in me speaking but you really should visit our coast (you are coming in the summer time, right?), you will love it. There's a lot of festivals, clubs etc. for you to enjoy. And you can even get to Montenegro if you follow the coast to the south. Visit Zagreb, Pula amphitheatre, Split palace and Dubrovnik Walls.
Hungary - Don't know that much about it so I won't comment really but I think you should be fine with 60-70 euros a day.
Slovakia/Czech Republic - I shouldn't be putting them together but they are so damn similar. You can get cheap drinks in both, have a lot of monuments to visit, both are quite nice and also not that expensive but will probably be a bit more expensive than they usually are because of the Euro 2012 that's in June, taking place in Poland which is right next to them. I believe it should be 40-60 euros a day depending on what you want to do. Take note that a lot of great sights/monuments in these countries (Poland as well) are not really close to bigger cities so you'd have to pay bus tickets for day trips.
Poland - I'm going there in June and then I'm going to Czech Republic after that. Be sure that it will be quite expensive because of the Football Euro Championship. I believe 70-80 euros a day will be enough but make sure you book your hostel early.
Austria - Austria is always nice but since summer is off season, you'll get good deals on everything, from hostels to food. 60 euros a day at max.
Also, do tell what you want to see and do so that I can reference that in my answer.
Switzerland I've never been but my Mom has and she was smitten. She did visit it in the winter months but it's suppose to be nice all year round. It's a bit more expensive than rest of the West Europe but you can count on spending the same amount as in France since you're staying with you friends.
Germany I haven't been to Germany as a tourist, just passing through and went to a music festival once so can't really comment on that. I don't think you'd have to spend more than 60 euros a day.
Denmark/Sweden/Norway/Finland Very expensive for tourists. Hostels are almost always amazing but are about 30-35 euros per night (in a dorm). Food is great, especially fish dishes but is expensive as well. You'll probably spend the largest amount of money here but you can always shorten the trip since it would be your last leg. You probably won't use all of it but put 100 euros a day for these countries. Also, you must visit Stockholm metro, it's amazing!
Since you mentioned Soviet architecture, how about you visit Belgrade in Serbia? It has a lot of buildings similar to Soviet ones. BTW, why aren't you visiting Moscow? It is pretty expensive but two years ago, I visited it and got a hostel room for 6 people for 8 euros per person per day. The old skyscrapers are amazing and you would enjoy taking photos of them.
But don't worry, you'll be able to see that kind of architecture in Warsaw, Czech republic, Helsinki and a couple of other places as well...
BTW, when you get to Zagreb, check out metal bar Runa, a nice bar in the Upper town (the oldest part of the town).
That's it from me, if you have any other, more specific questions, create new questions and we'll answer them as soon as possible if we know the answers!