I am reminded of what a Yosemite park ranger said when asked what a visitor should do if they only had an hour in Yosemite Valley... "I would sit on the bank of the Merced and cry."
Fortunately a week is significantly longer than an hour. Have a great trip!
When I was a young tad I had only a Pentax Spotmatic and a 55mm lens. Back then I knew exactly what camera to take.
Are you going to Cuba to take photographs or to experience the culture and people ?
Maybe it's because I'm getting older but I really don't understand the need for a backup or multiple systems on a short trip. If you're working, yeah, prudent decision but a vacation ?
Bring your Leica, take some photos (b/w to be different, how many photos of colourful old cars do we need to see ?) and if the camera gets cranky, enjoy the place without the filter of a viewfinder.
Chances are, you'll probably go back if this trip is great and then you can finesse the gear selection for that trip.
Have fun and bring lots of things to leave behind as gifts.
If it were I, I'd take my Contax G2 kit (G2 body, G1 body, 21, 28, 45, 90 lenses) and all the film I could carry. So in your shoes, I'd take the Leicas. Sure, there will be some shots you can't get with them - closeups of flowers and insects and the like - but there would be plenty of others you'd miss if you dragged along multiple camera systems and were fussing about which system to use for a given shot. The handiness of rangefinders plus the lack of a mirror letting you hand-hold in low light make them the best travel cameras.
When I travel with my camera, I don't want it to weigh me down. My advice is don't be a slave to your gear. Fatigue from carrying heavy gear can drain you of your creativity. I'm jealous. It's one of my dream destinations. Darn that Helms-Burton law!
Apparently about 100,000 folks a year, from the US, go to Cuba from Canada or Mexico. Just ask to not have your passport stamped by Cuban customs and take cash, don't even try to use a credit card.
Our travel agent said that many believe Cuba will be open to the US in the very near future and if this should ever happen the prices in Cuba will skyrocket. She suggested to go now!
It isn't so much about the camera as it is about seeing the shots. If you have too much gear to choose from, you'll busy your mind with thinking about what camera or lens to use, which in my opinion is counterproductive. My best results always come from sticking one lens on one camera, and force myself to work with what I've got. It allows me to let my imagination work full time, and to be ready when the moment approaches.