I continue to use WiltPruf despite a number of studies purporting to "prove" it does no good. The leaves of sprayed plants, in my experience, emerge from winter looking much better than those that have not received such treatment.

WiltPruf and other anti-desicants will not, however, do anything to protect against severe cold. Z5a is a very difficult environment for all rhododendrons. The three varieties you have are not very well suited to temperatures in the -20 degree range, despite what you were apparently told. Rosebud and Stewartsonia are Z6 at best evergreen types. Purple Passion has been highly touted as very hardy, but unless there are two different varieties circulating under that name, I have found it to be extremely prone to winter damage - so much so that it is not worth growing in my Z6a location. It is also very early for next year's flower buds to be visible, so what you're seeing is more likely to be leaf buds.

Anything you can do to protect your plants from winter sun and wind - burlap screens, evergreen boughs - will help, but will not increase the genetically determined limits of survival after deep cold any more than WiltPruf will. Withholding water from newly moved plants will not improve their hardiness. Insuring that they enter the winter with adequate water will. Adding mulch in the late fall will also protect the roots and delay freezing, but can be an invitation to mice and voles.

There are rhododendron varieties suitable for Z5. Hybrids produced by the late David Leach are among the best. Don't be overly discouraged by your experience with the three varieties you have been led to believe will grow well in your location.

Would it make sense to move the purple passion to a container?

Purple Passion supposedly grows to 6 feet in 10 years, so growing in a container would be fairly impractical after a few years. In the short term, there's no reason why it wouldn't work.

great advice. Maybe I can save the purple passion by potting. I will probably pot up the smaller plants and bring them into our home for the winter. Would it make sense to harden off in the garage, starting in March perhaps, to chill them down enough to bloom?

You would be much better off putting them into the garage in late fall or early winter but before it gets really cold in your area. They will have already set their flower buds for spring 2017 by then. Trying to keep them in the house will not work. The warm, overly dry atmosphere will cause them to abort the flower buds and will be harmful to the plants in general. If you can, wait until some time after Thanksgiving then move them into the garage. They will need no light at this point, so there is no need to place them near a window, etc. Put the pots close together on the garage floor, wrap around the pots with bubble insulation (probably not necessary, but easy to do and cheap insurance) and place a large cardboard or styrofoam box over them. This last will trap some ground heat. They will remain dormant and safe from severe cold, wind and sun. Check once in a while to make sure the planting medium hasn't dried out completely. A few mouse traps under the box is usually a good idea. Gradually remove the insulation, etc. as things warm up and increase exposure to light. Put them outside on sunny days but be prepared to put them back in a warmer area for the night during cold snaps. The Rosebud and Stewartsonia ought to bloom profusely with this treatment. May take much longer for the Purple Passion to be mature enough to set buds.