I think I have answers to some of your questions.
Personally, there's not really any good direct fast train service between Paris and Barcelona - most likely a fast train from Paris to the south of France, and then a regional/slow train the rest of the way, as the overnight train is a "regional/slow" train. If you're not worried about getting there quickly (but rather overnight), well, then that might be a good option for you.
You're looking at (from Paris) an 9-12 hour train ride. The EuroNight train is about 12 hours. There are many airlines that fly from Orly or Charles de Gaulle, and the flight is about an hour in length. The flight information is easy to come by, so I'll give you some more info on the train.
From Marne la Vallee-Chessy (
Disneyland Paris essentially for the train), you either could take the train back to Paris and do the EuroNight/TrainHotel, or you can take the TGV train south to Orleans, and then the EuroNight/TrainHotel train to Barcelona. Another option is the TGV connecting to a slower train to Barcelona during the day. The TrainHotel arrives around 8:30am, so I think it's easily feasible to do this and still get to the port in plenty of time. If you want to get there the night before, there's that option as well.
The night train is fairly safe from my travel experience. They also offer "premium" accommodation - so you can get a first class sleeper with shower/toilet, or just a reclining chair. There's also tourist class bunks that are 4 to a compartment (although no real amenities like "premium" class other than a sink). I don't know what your comfort level or budget is, so I can't make recommendations, but I would suggest looking at
http://www.seat61.com/ for further details. While it's focus is on the UK traveler to the rest of Europe, there's much good information to be found.
One site I like to recommend to get an idea of the train schedule is the Deutsche Bahn website, mostly because it's the most manageable in English (compared to the French SNCF website), and you can get an idea of what you're looking for.
http://reiseauskunft.bahn.de/bin/query.exe/e is that link.
For Paris: You can just select "PARIS" and it'll then break down all the major train stations in central Paris. Marne la Vallee-Chessy for Disneyland Paris. For Barcelona, you're looking for Barcelona Estacion de Franca (if you're doing the EuroNight/TrainHotel especially). Estacion de Franca is right down by the docks and Barceloneta, so I'd say maybe a 10 minute taxi ride or less.
Regarding Disneyland Paris, I've only stayed at the Disneyland Paris Hotel itself. With security changes, if you're arriving to DLP Hotel via the train station, ask the security guard which tent has the luggage scanner as you're arriving. We went to one (after much jetlag) to be summarily told there was another queue to join so that our baggage could be screened before walking to the hotel. I believe it was the one to the far right, but my recollection is a bit hazy from that early April morning in 2009. DLP Hotel is also the closest to the train station, and is simply a fabulous experience not to be missed.
Regarding tours in Paris, alas, no. However, I like staying in the Marais, that way, if I go to Disneyland Paris, it's an easy trek to the Chatelet Les Halles for the RER (suburban train) to DLP. There's many fabulous hotels near the Centre Pompidou, with good access to buses and the metro, as well as an easy walk to the Hotel de Ville and Notre Dame.
Enjoy your time!
-Steve