Charlie Pellet.
They can show up whenever. The best time is during rush hour, but they’re running on random trips throughout the day. An M3 was running on the Hempstead Branch all day on Sunday, so they’ll be running more frequently as service to GCM picks up.
7171 only has one side of the signs left, that's probably what you saw. They removed one of the signs on the car and kept the other one (don't even know why 🤷♂️)
As for 7501-7510, yes they still do. (Ironically 7505 is paired with 7406 and 7506 is paired with 7405 at the moment)
They started losing them eariler this summer. Word going around is that they're being taken off to prevent being stolen from vandals. There's only one set with the LEDs running around.