Dan, looking at your old examples, I think you have a combination of some sag (not a lot, it doesn't look as bad as I had with the ASI071) and somewhat incorrect spacing. The thing that I think is making this hard to diagnose is those two factors are combining to give you very inconsistent stars throughout the field, making it difficult to really figure out the direction of the tilt, and the tilt completely throws off the star angles in the corners, making it difficult to figure out how off your spacing might be.
If SV really does require a 0.5mm spacing or less to get good results with the reducer, then while you might need another spacer or two, I think what you also want is a set of adjustment rings. Adjustment rings come in packages of a dozens or so, with a couple rings at each thickness. The thicknesses range from 0.3mm to 1.5mm or so. You can add a ring between a couple of spacers or a spacer and the focuser and/or camera, to finely tune your spacing.
In order to really resolve the spacing issue, I think you will want to tackle the tilt issue first. It's not a lot of tilt, but it seems to be just enough to produce a very confusing issue. I couldn't really say more about where the tilt was coming from, not without handling the scope myself. I find that feeling for wobble or movement, with very light pressure at locations you think might have tilt, is the best way to identify potential problem points in your imaging train. If you think the focuser might be a source, that's where I would start. The simple fact of the matter here is...to fix your tilt, you'll need to play with the scope...and that means you'll have to deal with potentially worse stars until you get the tilt sorted. Also keep in mind...there could be more than one source of tilt. You might have a little sag in the focuser, you might have some tilt at the focuser/scope interface, you might even have some slight sensor tilt. It could even be a filter that isn't entirely seated properly in it's retainer or in your filter wheel...a darn dog hair caught between the edge of the filter and the rim in the filter wheel could actually be enough to introduce some slight tilt.