The only time the white object is officially mentioned is in the probable cause affadavit, in point 13. It doesn't say it is on the seat. It says "there appears to be a white in color object toward the passenger side front seat as it passes". This is from the image at 12:42 heading south on Beechdale road.
Yes, exactly this. It appears that this is the point LE are making in the affadavit; in point 19 where it describes the seconds leading up to the abduction itself, it describes what is presumed to be Linda as 'a lone person walking on Beechdale Road wearing white' (at 12:40:50). Further along the same point 'person wearing white' is repeated twice more. So imo, this is LE attempting to demonstrate that the 12:42 image caught a glimpse of Linda (person wearing white) just seconds later in JS' car in the 12:42 image - since they don't have footage of Linda actually entering the car, this is important to their case.
As to why the business owner at Harvest Structures took photos of the car interior, and what they showed, we have no idea - nothing concrete has been said about that.