1) Will the reruns continue on as they always have, or will the show be completely different now?
2) Can someone please explain Don Knotts' character. What was his goal? What was his motivation? The film seems to be a little uneven with his story line.
Actually both you and johnlennon18 did miss things....a LOT of things.
1) The reruns would indeed be changed. Don Knott's character had already told David that he'd already been observing the changes in the reruns since David took over for Bud.
Therefore the reruns had already changed, and as we see in the final scenes, things continued to change in Pleasantville. They are now in color, Mary Sue has gone to college and Betty is either no longer with George or is now with both George and Bill.
2) Don Knotts character: The TV repairman while fulfilling the "mysterious stranger" trope usual to this kind of story also acts as fairy godfather to David by magically granting his wish to live in Pleasantville. As he fills this kind of fairy godmother role, no need to ask his goal or motivation. He's a magical plot device.
Whatever the repairman's origins was, he was a lover of tv and was familiar with probably every show ever made. He created a magic remote that could send people into the reality of tv shows, but he had been searching and searching for someone like David with enough knowledge about some show to be a prime candidate for testing his device and seeing what happens. I don't think he intended for his sister to end up there too, it just happened cause they were both holding the remote. He indeed was a mystery left to our imaginations.
Despite the answers below I don't think there's any way to determine these outcomes, as the special remote Don Knotts gives them seems to indicate some sort of time-space alteration. The lack of knowing Don Knotts motivation actually kind of bothered me also. It doesn't matter, it's a good film that leaves you to make up whatever happened in your imagination.