I am against trimming the root systems, all the energy and nutrients stored in the roots for the following spring are lopped off. The first group of vines I put in I followed those instructions and those vines suffered there first few years. Hopefully yours do not.
Anytime before the second season is fine, no right or wrong. The first acre I did it first, while the second acre I did the following year. It is easier to get a tractor in and out doing it first.
There are also a few different apps that track GDD, Growing Degree Days by iNet Solutions Grouphttps://itunes.apple.com/us/app/growing-degree-days/id386655475?mt=8
I initially bought a sprayer with multiple spray heads sort of like the first mist sprayer, it didn’t work. The problem is the the spray does not penetrate the canopy enough to protect the crop. The second sprayer might work. If you are doing 2 1/2 acres, bite the bullet and get an air blast...
Muscadine vines thrive best in 5.8-6.5 ph range. I would say you need to try to lower your ph. Sulfur works well if you can till it in around the plants. This may damage the roots a bit, but might be worth it in the long run. You could also fertilize with an acidic fertilizer or use peat moss