BA has me booked with a 70 min transfer time , all in Terminal 3. Arriving from Spain, departing to USA. Is this legal or even possible that it will work?

Shouldn't be a problem. They won't book you on flights that don't meet MCTs (minimum connection times) as established by the airlines and airport (1 hour BA > BA at T3.) Follow the signs to flight connections, go through a security screening, and you'll be discharged into the T3 shopping mall.

That should be fine. Don't dawdle, but it shouldn't be a problem.

Another change to dlight itinerary. My flight from domestic arrival to international take off at ORD has now been feduced to 55 minutes. Is this doable? Same terminal. May carry on luggage as I usually do.

Same answer, no problem.