It is really sad that people continue to take bees into previously Varroa-free areas. Anyway, you asked for comments on your plans.

The timing between treatments - why not repeat twice (or more) at roughly 6 day intervals? Then you've covered a full cycle of sealed brood even if the queen is laying. A few of my crown boards have some detectable warmth so I think some colonies here are brooding. Of course, I can't recommend multiple sublimation, as the only legal way to treat is to use ApiBioxal according to the instructions, and they say sublimate only once per year. For OA treatments I just treat by the calendar rather than after cold weather. Daylength seems to have more influence on brooding than recent temperature and colonies will just briefly restart brood rearing at random times no matter what the weather.

Your results are probably proof that icing sugar does not work as a Varroa control. There have been papers published on this. In much of the year most mites will be inside sealed cells and therefore immune to powdered sugar. Even those out of cells may not be affected. Some mites are firmly clamped on and lodged between abdominal segments so I doubt that a high proportion of them are susceptible to the icing sugar. Use it only to monitor mite levels, and monitoring that way is much better than looking for mite fall onto a sticky board ... and that is better than mite fall onto a dry board.

Finally, the University of Sussex's study is just one study amongst many. Even that study doesn't report a significant difference for most parameters between trickling and sublimating. Colonies do seem stronger in spring after sublimating but they used a standard 50 ml for trickling which is significantly more than most of us would use - so it wasn't a fair comparison. However sublimation is a good method so no need to change now.