If the plane goes supersonic at 10 miles of altitude, will it still make much sound on the surface? It's not just distance, it's the pressure of the air at the altitude, too.

Haven't engines improved a lot since 1967?

No idea about cost, but currently oil is cheap and abundant, compared to 1970s, and the U.S. has a large domestic supply.

Regarding identical planes, I suppose first Boom's supersonic planes are going to be mostly identical. But even standard airliners get small changes with every dozen planes built.