re: 2, if there’s few or none people around, the beacon slows down the advertising, so the responsiveness impact is the same is if you’d increase the advertising interval. So for example, it might take a few more seconds to detect an enter, or distance estimations might be less reliable for a few seconds after you start ranging, as it takes a few seconds for the beacon to speed up the advertising back to the regular value.
re: 3, Tx power is not affected by the Smart Power Mode.