Is this Bethesda MD? And you had an older home torn down, and a new home built?

I'm afraid I don't really have anything helpful or constructive to suggest, but I wanted to say that I'm really sorry that happened to you.

Do you know what kind of joists your floors are supported with?

I am so sorry to hear this. I wonder if this is one the homes in my neighborhood? Who is saying that the new home warranties are null and void? The builder or the manufacturer of the products? In Maryland, you can report a builder to the licensing board. Moco government is also really tough on builders. What has been the communication about the issues to the builder? How long have you been in the house?

Regarding the floors, do you know what kind of underlayment they used? I am not sure that blocking the joists will add much to the effect of movement and noise. You may need to tear our the whole floor and start again.

Have you had a consultation with a lawyer to explore all you options?