There isn't any air between the film and the pressure plate to expand outwards - heck, if there were the film would have to be bowed out all ready. And as the bellows is opened all the air in the camera all looses pressure at the same time: so the air pressure around the pressure plate (and in back of the plate if it has a hole for a red-window) is the same as the air pressure in front of the film so even if air were claimed to rush in behind the film at the last minute it won't.
The film in some cameras is backwound around the rollers - again Hasselblad comes to mind - and the film can have a bowed out crease if it is wound on just before a shot. The second shot can have a crease if the camera had been left alone for a while even if the film was kept wound on. Most (all?) folders have a straight film path so this isn't a problem.
My practice from 50 years of shooting with folders and other cameras without double-exposure-prevention is to wind the film right after talking a shot, then the camera is always ready to take a picture and there is never any question of 'did I wind the film????'. I may open and close the bellows several times in anticipation without taking a picture. A double exposure is a very sure way to ruin a photo.
You can check infinity focus by aiming at an infinity target, setting the lens at infinity and checking focus at the film plane. A bit of Scotch frosted sticky tape stretched across the rollers makes a good temporary 'ground glass'. The lens should be at infinity, the image in focus and the rangefinder in co-incidence if all is copacetic.