1. yes. In the re-fueling era ‘burn laps’ were used to adjust strategy pre-race.
3. only change what they can do in a pit-stop, except for tires. ie. wing adjustments etc. any significant body change from parc ferme (and this is defined in the regulations) would require the car to start from pit lane
these early laps are installation laps, they run through a check list of all race systems (drink, radio, engine map, clutch settings for the start, under/over steer etc. etc.)