Just to update the thread, the two hour layover both was and wasn’t enough. The problem was that my flight to LHR was delayed by 90 mins. This would have left me very little time to get to my next flight, especially as it was the other end of the terminal - i used the train shuttle. Fortunately, that flight had also been delayed by 2 hours so the problem disappeared. There were no controls or anything beyond checking boarding passes. Immigration at Dallas was a doddle. they checked my passport and that was it.
And yesterday i was sitting on a sun-lounger beside a pool sipping white wine while watching the eclipse.