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Martin, are you solely relying on the laser to collimate your Newt? If so, there's 50% of your problem solved. A laser will not help collimate the secondary mirror. With the laser, you will not be able to determine if the secondary is square with the focuser. The laser may "say" that the beam coming off the secondary is pointing to the centre spot on the primary, but because it is a point source of light, it is not telling you if the secondary is properly aligned to the focuser.
What you are seeing of the stars being blurred on one side is a tell-tale sign of a misaligned secondary mirror. I too thought that a laser was a "do it all" tool, and instead of helping over time things were getting worse and worse. And the skewed star image was one thing that just refused to improve. And it was after I exhausted everything else, I thought I'd look at the secondary mirror for no particular reason, and boy didn't I get a surprise! It was WAY off. Worst thing was I had sold my Cheshire eyepiece when I bought the laser. I've come to find out that a Cheshire will actually deal with not just the primary but also the secondary.
Flex of the OTA would be noticed as a change in shape of the stars with the OTA pointing in different directions from horizontal to vertical. You don't appear to be finding changes in star images. With a heavy camera such as your DSLR, this flex would be most obvious as the scope changes in its orientation.
Similarly, overtightened tube rings wouldn't be a problem because you "collimated" the scope after setting it up. This would not be the source of your blurred stars.
Another tell-tale sign that it is your secondary mirror is that this problem is getting more pronounced. If it was flex then this problem would have been obvious from the start.
Alex.
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