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Most people sterilize their containers by wiping them down with a weak bleach solution. This article should also help: https://www.gardeningknowhow.com/special/containers/how-to-clean-containers.htm

There are actually many wildflowers that can be successfully grown in containers. You need only provide them with adequate drainage and keep them watered, especially during hot and windy weather. As for their seeding, you can usually deadhead the spent blooms and dispose of them to prevent this. Here is more information: https://www.gardeningknowhow.com/ornamental/flowers/fgen/container-grown-wildflowers.htm

You can use any kind of barrel or tub, as long as it has some drainage, and did not contain a poisonous material. This article will help you get started with container garden: https://www.gardeningknowhow.com/special/containers/novice-container-gardening-tips.htm

Are you wanting containers or a raised garden? With containers, you can put them anywhere and there's no need for placement of any wood. On the other hand, raised beds will require wood or some other form of edging to keep the soil intact. These articles should help you: https://www.gardeningknowhow.com/special/containers/novice-container-gardening-tips.htm, https://www.gardeningknowhow.com/special/urban/raised-beds-for-urban-settings-no-digging-required.htm, https://www.gardeningknowhow.com/edible/vegetables/vgen/raised-vegetable-gardens.htm This article will provide some suggestions for keeping animal pests away: https://www.gardeningknowhow.com/plant-problems/pests/animals/camouflage-gardening-deterring-garden-crashers-pests.htm Sorry, we do not sell fencing; however, a home & garden supply center, like Lowe's or Home Depot, should have what you're looking for.

While I cannot say with certainty, I believe that it should help insulate the plant roots from extreme heat and cold but will still require plenty of watering. Container grown plants need to be watered more often, regardless, especially in high heat situations. So, adding a foam liner will help but keeping the plant watered, and even mulched, will help prevent it from drying out faster.

Generally, this type of mulch is most often used for play areas and similar locations, as there may be some concern in chemicals leaching into the soil around plants. There are, however, many different types and forms of mulch available. These articles will provide some basic information and help with deciding which type may be best for you and your needs: https://www.gardeningknowhow.com/garden-how-to/info/learn-about-synthetic-mulch-for-your-garden.htm, https://www.gardeningknowhow.com/garden-how-to/info/whats-the-best-mulch-for-my-garden.htm