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As long as the plants you had in it before were healthy, there's no reason why you couldn't use that soil. However, you may want to take it out and refresh it by mixing in new soil or compost.

When gardening in a container, it is best to use a soilless mix. This will help prevent compaction, which makes it difficult for water to get to the roots freely and for the roots to grow well. You can either buy a soilless mix (aka potting soil) or you can make your own. This article will help you with making your own:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soilless-growing-mediums.htm

It sounds like you've done all you can thus far. There's not too many plants that will survive winter, so you may just want to wait for spring planting.

Soil from your yard will compact in containers, which will make it very hard for your plants to grow a healthy root system. This happens because of 2 reasons.

First, garden soil contains fine ground rocks that naturally fit together. Think of how sandstone is formed and the soil in your garden is what sandstone was before in compressed.

Second, out in your yard or garden, there is a thriving, complex ecosystem that is continually aerating the soil (preventing the compaction from the first reason). Things like burrowing critters, like earthworms, bacteria, decomposition of organic material and water make sure that most soils in the garden stay uncompacted. Clay soil is the exception because it has a high, fine mineral rate, which is why you need to add organic material to clay soil.

In a container, the area is just too small to maintain the kind of ecosystem. There is nothing to stop the minerals and ground rocks from coming together. And roots have a difficult time growing that way.

When filling your containers, you don't need to use synthetic. You need to use organic material only. Compost, well-rotted manure or peat moss will work well, as will store bought potting soils, which are typically made up of a combination of these materials.

Try neem oil. Very effective and not harmful to people or animals. Here is more information:
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

That is not really a question we can answer. You would need a builder come look at your house and terrace to check how much weight the structures could take. That being said, this article on terrace gardening may have tips to help as well: https://www.gardeningknowhow.com/garden-how-to/projects/building-terrace-gardens.htm

As far as the place staying clean, most people do not have much of a problem with the space getting dirty from garden containers.