If you stop a car on a sidewalk, do you have to ensure that pedestrians have a passage of at least 2.5 m wide?

No, the regulations require leaving a pavement width of at least 1.5 metres for pedestrians, not 2.5 metres. According to Art. 47 para. 2 of the Road Traffic Act, stopping or parking on the pavement is permitted, provided that a space of at least 1.5 m wide is left for pedestrian traffic.