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Late May, after the danger of frost has past.

I recommend cutting only what is dead or injured right now. Thin out what you can, and possibly fertilize them with an all-purpose fertilizer once or twice a year. Here is an article with more information: https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/dahlia-care-grow-dahlia-plant.htm

Store as you would, normally. It would be a good idea to give it a dab of peroxide there to keep away potential infection. Here is an article with more information: https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/storing-dahlia-tubers.htm

I recommend leaving the new growth until it dies off naturally, and then you can cut it off. Best to let it store some energy while it can before it goes dormant. Here is an article for more information on the care of these: https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/dahlia-care-grow-dahlia-plant.htm

It is not recommended to do this. They are prone to rotting, and you may find that by the time you need them to grow again, they might have completely disappeared.

Here is an article that will give you instructions on how to winter these properly: https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/storing-dahlia-tubers.htm

These are pretty picky with how they like to be stored. Almost any change will cause them to rot. This article will guide you to storing these properly:

https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/storing-dahlia-tubers.htm