Thank you for your questions, Jeanne.  The worst thing you can do is to prune during hot, dry weather.  It just creates wounds that allow moisture to escape.  And removing more than 1/3 of the plant's foliage each year deprives it of enough leaves to photosynthesize adequately.  Use a soaker hose to water daily, early in the morning.  Cover the plant with air permeable white fabric, such as a gauze, to protect it from the sun's rays.  Mulch with large bark chips to 2 inches depth.  And keep your fingers crossed!

I suggest that you buy an inexpensive soil water meter (probe), to measure how deep the water has permeated.  There's no "perfect" time, since soils vary in content and drainage rate.  These roots go down 1 to 2 feet, and are fibrous, so don't have a tap root to find deep water.  Here is more information:  https://www.rhododendron.org/irrigation.htm#:~:text=They%20have%20fibrous%20roots.,and%20into%20the%20surrounding%20soil

Good luck!

Welcome!

You can prune off dead, diseased or dangerous branches/tissue any time of the year.