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Unless the leaves are severely injured, or blocking light from a portion of the plant I wouldn't recommend cutting these off. Doing so will not kill them, but the need to make enough energy to keep themselves alive. These large leaves make a lot of energy, so unless they are preventing another main growth point from receiving light then it will only slow them down to remove these.

Can you include a photo, so that I may be able to see the plant? This will help me to determine whether cutting the leaves off will hurt or help.

Tiny black creatures sound like aphids. You probably don't want to blast them with the hose; the force might break a stem. Insecticidal soap or neem oil should work well. If you don't think they are aphids, please write again with detailed description and exactly where they are on the plant. A picture works wonders. Here is more info on aphid control: https://www.pesticideresearch.com/site/pri-resource-centers/pest-mgmt/pest-mgmt-bulletins/aphid-control/

You can prune away the dead plant material and continue to care for the plant. I see lots of green growth still on the plant.
This article will help you.

https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/grow-dahlias-in-containers.htm

Probably no great harm to leave it on the plants short term until you get rains that will wash off the ash. But if up to me I would hose off a heavy deposit on and under leaves because it can block sunlight access and photosynthesis of sugars that nourish plant tissues, and it can block stomata that are openings for leaf moisture regulation and respiration. It may also have a drying effect.
Heavy deposits of ash can affect soil chemistry and mineral balance, sometimes good sometimes not so good. After the fires are done and the winter rain and snow has taken the ash into the soil, you may consider running a soil test in spring to analyse soil mineral content and pH (acid/alkaline balance). And correct imbalances with recommendations from a soil analyst.

This article will help, actually!

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

Hello,

This may be a case of different soil types. The soil in which she planted hers may either have more nutrients available, or a more stable pH.

Add some dolomitic lime to the area around yours, and I bet that solves the issue. My guess is that the pH is out of range.