max105, welcome to GFE!
Your proposed Itinerary looks fairly well organized, but I'd suggest a slightly different number of days in each city. The first thought that comes to mind is that not enough days have been allocated for Paris. Even though you've been there before, it's an important city with
lots to see, but you've only allowed about a day and a half?
One other thing to mention is that the cities you're visiting are all relatively close, so you won't have to use a full travel day to get between them, especially if using the
Thalys or other fast trains. You might enter all your rail trips into
www.railsaver.com as it would be interesting to see what combination of rail passes and P-P tickets the site recommends?
To answer your numbered questions:
1.) I'd probably tend to focus on Bruges and just visit Brussels for a day trip. I'm sure the others will have some suggestions on that as well.
2.) I would definitely buy the train tickets at the station, rather than pre-booking as it's usually less expensive that way. I usually purchase my tickets a day or two before I'll be travelling. Note that some tickets will include compulsory reservations (this usually applies to the fast trains such as the
TGV).
3.) As mentioned above, I'd probably try to add at least a bit of time in Paris.
4.) If you'd rather forego Paris this time, Normandy is certainly an option. However, keep in mind that the D-Day sites are quite "spread out" so it's difficult to get to the beaches unless you have a car
or unless you take a tour. If you'd like to take a tour, I'd highly recommend
www.battlebus.fr. They're a bit on the "pricey" side but they provide
incredible full day tours! Two things to note if you'd like to consider this - first, it will be necessary to spend at least one night in Bayeux, as the tours depart from there promptly at 08:00 (you could also see the famous Tapestry while you're in Bayeux); second, it will be
very important to make reservations as especially as even at that time of year, they could be booked up. Keep in mind that this year is the 65th Anniversary of the landings, so I'm sure people will be starting to trickle into the area in May to get ready for those, and lodgings will be at a premium.
For Normandy, you'll also have to allow some extra travel time to get to and from that area. You'll be travelling from
Gare St. Lazare, so on the way back you'll have to allow enough time to get to Paris / CDG for your flight home. That should be a straightforward trip - train to Gare St. Lazare, Metro to possibly Chatelet les Halles and then RER to CDG.
5.) I'd have to do some checking to find out whether your prices are "fair". It would help to know where you got the amounts for Hotels? If you haven't checked already, you might want to have a look at a few Guidebooks as they usually have lots of listings in different price ranges. If you want to reduce the costs for lodgings slightly, you could consider Hostels.
I'm sure the others will be along shortly with some comments and suggestions.
Good luck with your planning!