What's your question? Ask

If you are hesitant, you can wait till early spring to cut back the plant. At that time, the new wood will be green in color, while the old wood will be brown. You can use this rule of thumb for fall/winter pruning too, but it is much more obvious in the spring.

Yes, you can change the color at any time. This article will help with how to do that: https://www.gardeningknowhow.ca/ornamental/shrubs/hydrangea/change-hydrangea-color.htm

It normally takes about a season to change the color.

As long as the stems are still green, it should recover. I would not trim it back yet, the leaves may regrow where the old ones died back. Give it a week or two and see if you see leaf buds forming before pruning it back. That being said, you may not get blossoms this year as the frost may have killed the buds.

It probably cannot be repaired, but the steps you have taken are what you can do to at least give it a chance. Don't write them off yet as not being able to achieve the same size. They can fill in rapidly, so in a year or so, the size difference may not even be noticeable.

Have the soil tested. It may be a phosphorous deficiency. Purple leaves are a sign for this deficiency. Regardless, adding a little phosphorous will not hurt the plant and will help with blooming. Here is more information: https://www.gardeningknowhow.ca/garden-how-to/soil-fertilizers/phosphorus-plant-growth.htm

I think the fungus you are referring to typically shows itself as purple spots, rather than whole leaves turning purple. But, if you are still concerned that it is a fungus, treating the plant with a fungicide will not harm it. You can buy fungicides at your local plant nursery.

I would not prune them right now. Most hydrangeas bloom on old growth, so if you prune, you will lose the blooms next year. They will regrow their leaves when spring comes, as long as the stems are alive. I would wait until spring and once you see what stems have died, you can trim them away.