Honestly I think if you want something just ask for it and they'll do their best to make it happen for you. Just tell them that it's important for you and that you chose the Blau based on guest reviews and their service.
From what I've seen there is live music during the dinner, at least in the photos I've seen. You can probably mention it to her and see what she says.
I've seen photos where people have done their first dance right in the restaurant and I'm sure you can do your speeches there as well. There is also a disco by the beach where you can dance afterward. It's supposed to not be very busy which could be good because you can give the DJ a CD to play. That's what I plan on doing, slipping him a few pesos and having him play our first dance song and whatever else we might want.
As far as chairs on the beach there was discussion about this earlier in the thread. I think the resort is reluctant to put down chairs but they will do it if you insist on it. What did Isabel say when you mentioned chairs?
We've decided to get married in the gardens and then just do photos on the beach. I think I prefer that because it's more private, they decorate it for you (and we'll bring some of our own decorations as well like poms to hang from the trees) and it's also more secluded from the wind (it can be pretty windy in Cuba in December and January).