I believe you can cancel air before final payment (anyone else confirm?) so maybe book airfare with Disney while you explore the options. If you set up a travel alert on Kayak or other site, you can watch for drops. Check google flights for options on flights and if there is one you like, sign up for that companies flight alerts lengthier website and if there isn't a huge price difference use their site and not a third party (example expedia) to book. Make sure you check for options (liked a checked bag and seat selection) if these are important to can we the cheaper fares usually don't come with these anemities.
I don't recommend booking into domestic flights then a flight to Europe, as any flight schedule changes can ruin your perfect planning. Book on one ticket so if there is a problem with one leg it won't effect the other (I think that is what happened to the Miami flight above-correct me if I am mistaken) or the flight will be changed/accommodated for free. If it is all one one ticket then then changes are made free, but on separate tickets you are on your own. I missed my connecting flight coming home from DulIes, but they put me on the next flight without any fees. (Weather delay). I had my Air France flight cancelled, but without any fee, I was able to move it to another day (two days earlier) to meet my needs because both were on one ticket.
If you are going to Barcelona, just book to Barcelona. If your are not a frequent flier, any booking tickets here to there and then on will just be confusing, stressful, and not worth your time and effort. Some of those "cheap" airlines charge a few dollars (euros) for a carry on, a few to print your boarding pass, more to eat, money for a beversge, money for a blanket and a few more $$$ to breath (well almost) so it may not be worth it.
But if you must search for options, try Madrid and do a few days pre or post there-you can get a train between there and Barcelona. (A little easier when you are in the same country and you could more easily book a flight between the too). Just research to watch for the fees and research carefully. Only once did it ever behoove me to get a cheap/discount airline flight (that was in Africa where it came with a checked bag but no drinks between Cape Town and Johannesburg.)
I have done nonstop, domestic stop, European stops, and two stops flights. The more stops the less $$, and the less stops or lay overtime the more money you pay. You just have to find the balance. Good luck!