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Fall is a great time to plant perennials. Even after leaves fall in autumn, roots continue to establish themselves in the soil. Everyone loves close out sales; however, these plants may have been in the same container since spring. There is a good chance that they are rootbound. It is important to straighten the roots as much as possible. If there is a tight wad of roots hopelessly tangled, there are 2 options. One is to soak the roots in warm water and see if the roots loosen. It is OK if the soil falls off. The other choice is cutting an X one inch deep through the bottom and 4 vertical slits around the rootball. Make the planting hole wide enough to spread the roots out. If lots of soil falls off from soaking, make a mound in the center of the planting hole and place the plant on top, spreading out the roots. Hold off on fertilizer until next spring. Keep the plants watered if fall is dry to lessen the risk of winter kill. This is true of all plants, especially newly planted ones. Even without leaves, plants lose moisture to winter winds. A dried out plant is a dead plant. Enjoy your new plants!

The closest thing I can find to this is mealybugs. The photo below shows some hairy projections above the group. If not that, it does seem like it is an insect problem. Try an insecticidal soap or Neem oil.
https://www.gardeningknowhow.com/plant-problems/pests/insects/mealybugs-control.htm
https://www.gardeningknowhow.com/houseplants/hpgen/common-bugs-and-pests-on-houseplants.htm

Almost all plants and flowers can be preserved. This article will help: https://www.gardeningknowhow.com/garden-how-to/projects/drying-flowers-and-foliage.htm

It is hard to say whether that is normal or not without knowing the plant in question, the environment in which it is kept, and other information that you can give. I'm sorry, I can't really give you an answer at this time.

I think this would be better suited for a contractor, or someone that will be knowledgeable on building, or carpentry than a gardener. We can help with the plants, or amending the soil, but we don't really offer any information on contracting and building.

You will have to contact a local contractor in your country, I'm afraid.

It depends on the plants and the size of the basket. I almost always buy too much and have to find another spot for my surplus. You want it to look full right away but then you need some growing room, too. Purchased hanging baskets aren't put out for sale till they are full and looking good. So when you do your own, the basket may look sparse for a short time till the plants fill out. For a 10 or 12 inch basket, 4 to 5 plants should be enough (in 2 to 4 inch pots). Here are some plant suggestion:
https://www.gardeningknowhow.com/special/containers/hanging-basket-plants.htm
https://www.gardeningknowhow.com/ornamental/flowers/petunia/hanging-petunia-plants.htm