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This depends on the metal! Iron, or rust in the soil can help as an acidifier, and to put iron back in the soil. Something like copper, or aluminum however, can be fatal to the plant depending on how much it breaks down in the soil, and how much the plants actually take up through their roots. As a general rule, I would not recommend putting metals into the soil in this fashion.

Late winter to early spring is a good time to start working on your soil, provided the weather is somewhat warm and the ground is workable. Here are some tips for amending soil that may help: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/improving-garden-soil.htm

Note, you may not notice the pH changes right away, but certainly within a month or so with careful monitoring you should notice a change. Make sure to test both the soil and pH beforehand to help determine what changes are necessary.

There are several options that you have here. This article will get you started with building soil: https://www.gardeningknowhow.com/special/organic/the-importance-of-soil-for-an-organic-garden.htm

I would start with a soil test. Check out your state's Extension Service; some states offer this for free. Problems are showing up in soils with too much organic matter and toxic, high levels of nutrients. Nitrogen moves through the soil quickly but phosphorus, for example, does not. Toxic levels depress yields. If done through the Extension Service, you will be given information on how to correct any problems. Check out this information: https://ag.umass.edu/soil-plant-nutrient-testing-laboratory/fact-sheets/corrective-measures-management-of-over-fertilized

No, to sterilize soil you need to raise the temperature to 180 degrees F.
Here is an article with more information.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/sterilizing-soil.htm

You can purchase sterile soil in bags. The label should clearly state if it is 'sterile'.

The company says this mix is less likely to get fungus gnats so they took out compost and bark or other "forest products". It also contains a wetting agent and up to 6 months of fertilizer. The fertilizer is high nitrogen, NPK ratios are 3-1-2. Espoma fertilizer for tomatoes has a ratio of 3-4-6 so you can see these are very different. You can supplement the indoor food (meant mainly for leaves and flowers) with additional phosphorus and potassium if you are using this mix in containers to grow vegetables. If you are using it just for starting plants indoors, it will be OK as is. You might find this article useful. http://www.weekendgardener.net/garden-plants/best-garden-fertilizers-060906.htm