It wasn't that much faster, I think it was 0.1 mach faster, anyway the SR-71 design was a much more versatile and a 2 seater and they really needed two pilots to get the missions done that they wanted efficiently, even though one wasn't really doing a pilot stuff. Anyway the SR-71 could carry more camera's and spy equipment then the A-12.
The A-12 was designed as more as an interceptor aircraft, or at least that is way it was sold to congress for funding.
You have that right. Congress being dominated by Democrats, may of them great believers in the UN and a "world police force" to protect us from the Russkies' aggression, it would have been a tough sell to promote the SR-71 as successor to the U-2. It needed a cover story.
Sane Americans had misgivings about basing A-12 squadrons in New Mexico presumably to intercept Soviet bombers over Alaska and northern Canada.
Of course, it was still anyone's guess just how fast the aircraft could fly in those days. After LBJ let the cat out of the bag in 1964, I would guess a lot of jaws dropped.