With a DVD it shouldn't allow you to go any wider than 720 as that's all there is to begin with. You won't get higher quality by making up pixels. The best you can get is to use what is there, 720X480 (or 576 PAL).
That was my next to last CATV job. Return path mitigation. Way too many possible causes. But you hit the main ones there. I might add that a lot of that bad cable and fittings comes from DIY installations.