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In container plants,  this is simply due to moisture retaining additive found in potting soil or bagged compost, which absorbs excess water and gradually releases the jelly-like substance as the soil dries. Nothing to worry about. It could also be due to some type of pest or fungus if found in regular garden soil. In this case, treating the soil with neem oil is helpful.

This smell is normally either caused by natural sulfur in the soil or by soil that has poor drainage (it will be swampy). If it is natural sulfur, there is little you can do to correct it. If it is poor drainage, improving the drainage will help get rid of the smell.

Being so long ago, it is really difficult to know for certain if there's still a threat or not. Your best best would be to either plant the garden far away from the creek (about 50 ft or better) or install raised beds instead. Here is additional information that may help: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/contaminated-soil-treatment.htm, https://www.gardeningknowhow.com/edible/vegetables/vgen/raised-vegetable-gardens.htm

Compost is one of the best ways to enrich poor soil. These articles should help with improving your soil and lowering the alkalinity: https://www.gardeningknowhow.com/composting, https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/what-is-soil-made-of-creating-a-good-garden-planting-soil-type.htm, https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/raise-acid-level-soil.htm

Here is an article dealing with soil tests and interpreting results. You might find it helpful: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm
Excessive phosphorous and potassium are not generally considered to be detrimental, because plants only absorb limited amounts of these.
No matter what your soil characteristics, addition of organic matter is always an improvement.
As to what you can plant, that depends on what you want -- edibles, lawn, landscape, flowers -- the possibilities are endless.

The trick with those is keeping their roots cool. Try mulching your brassica plants. This will help keep their roots cool, which helps to improve their performance.