On Delay of Game: check the NFL rules for which official is responsible for the playclock, IMHO it's the Back Judge together with the Referee. The only thing they have to do is anticipate better on the playclock reaching zero seconds when they look at the snap. And a signal like a buzzer or otherwise, would have to be sent slightly in advance of the playclock reaching zero seconds, to take into account the delay from the moment the clock hits zero to the moment the zero is registered by the eyes of the officials. I don't think we should overload the officials with electronic devices, just because once in while the human mind decides to react slower than usual.
On Measurement: The Umpire is the official that takes care of the placement of the ball. The officials on the sidelines decide to which point the ball is advanced to, and the umpire puts it at that spot. The ball doesn't move an inch until the center lines up again. In case of a measurement, the officials crew moves all players out of the way, and the chains are brought in, with the wing official holding firm grip of the chain on the spot of the best fitting 5- or 10-yard line. The chain is placed alongside the ball, and the back stick is placed on the ground (guided by the official holding the chain), and finally the front stick is placed down to see where the ball is in relation to the front stick. All this is standard officials work, I don't see where you get your assumption that they are just guessing, or do it "from memory"? One thing to remember: all the lines (yellow, blue, red, whatever) drawn on the screen are unofficial. The position of the ball is determined by how far the ball has advanced in the runner/receiver's hands, and a first down is determined by the relative place to the chain. And to get a first down, only the tiniest amount of the tip of the ball has to get past the front stick of the chain.