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I suggest spraying the plants with a product called Green Cure at 4 to 5 day intervals for at least 5 sprayings. Be sure to mix the product with water at the Cure Rate listed on the product label. Downy Mildew is a tough customer to get rid of, so you must keep after it. If things do not appear to be improving after the 5 spraying regimen. Then do yet another regimen of spraying. Another option would be spraying with a fungicide known as Mancozeb. It does leave a yellowish powdery residue on the plants leaves but that is part of how it works. Use the Mancozeb at 7 day intervals.

I think the leaf damage is most likely environmental or sun scorch from winter.
The damaged leaves and branches can be pruned from the plant.
Here is some links with more information.

https://www.gardeningknowhow.com/ornamental/shrubs/rhododendron/rhododendron-care.htm
https://www.gardeningknowhow.com/ornamental/shrubs/rhododendron/issues-with-rhododendrons.htm
https://www.gardeningknowhow.com/ornamental/shrubs/rhododendron/how-to-fertilize-rhododendrons.htm

Here are some articles to help with Rose care and fertilizer.

https://www.gardeningknowhow.com/ornamental/flowers/roses/rose-fertilizer.htm
https://www.gardeningknowhow.com/ornamental/flowers/roses/best-rose-fertilizer.htm

I would go ahead and repot the Evergreen now. Make sure not to use to large of a container. Just large enough to allow new growth.
Use good quality potting soil and make sure the container has adequate drainage.

If you water and fertilize this rhododendron, one possibility is overwatering or overfertilization- after pruning, it would need less of both, and too much could be harmful.

Cold winter temperatures could have also damaged your shrub. This may not have anything to do with pruning. If it still has some green leaves, it will probably recover.

It looks like the one remaining leaf has some black spots on it, which could indicate a Cercospora leaf spot infection. This could have caused the loss of the other leaves. Other diseases like Cylindrocladium blight or Botryosphaeria canker could also cause these symptoms.

In any case, you should remove any fallen leaves from around the rhododendron to prevent disease spread. Please see these articles for control advice and symptoms:
https://ag.umass.edu/landscape/fact-sheets/rhododendron-leaf-spot
https://extension.psu.edu/pests/plant-diseases/all-fact-sheets/azalea-rhododendron-diseases

This article will help you pinpoint the issue.

https://www.gardeningknowhow.com/ornamental/shrubs/azalea/azalea-not-blooming.htm

You can successfully grow a Rhododendron in a container.
This will allow you to enjoy now and take with you to your new home.

Here are some links to help you.

https://www.gardeningknowhow.com/ornamental/shrubs/rhododendron/rhododendron-care.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/acid-loving-plants.htm

https://www.gardeningknowhow.com/ornamental/shrubs/shgen/potted-shrubs-growing-shrubs-in-containers.htm