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If they are broad leaf weeds, you can use a broad leaf specific herbicide to help eliminate them. But if they are grass like weeds, you may not have any choice but to either hand weed or consider re-seeding the lawn. It sounds like you may have some underlying issues that are contributing to the loss of the desirable grass, though. Often when weeds set in like that, there is something about the soil keeping the desired grass from growing healthy and then giving the weeds an opportunity to get a foothold. You may find these articles helpful: https://www.gardeningknowhow.com/lawn-care/lgen/what-the-weeds-in-your-lawn-are-telling-you.htm, https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-types-and-weeds.htm, https://www.gardeningknowhow.com/lawn-care/lgen/tips-for-improving-the-lawn-and-reducing-maintenance.htm

Crabgass killer will take care of the Dallisgrass without hurting your St. Augustine grass. Here is additional info that may help: https://www.gardeningknowhow.com/plant-problems/weeds/dallisgrass-weed.htm

Unfortunately, this grass is not immune from grubs. However, if grubs should become a problem, this article has some suggestions that can help: https://www.gardeningknowhow.com/plant-problems/pests/lawn-grubs-how-to-get-rid-of-grub-worms.htm

There are no plans for a 'nationwide' ban on this, but there are some local governments in water stricken areas that feel it should not be planted. These localities will vote independently and you would have to check with your own locality to see if this kind of ban is in the works.

This is probably not a fungus, but the nefarious spittle bug. Here's some information: https://www.gardeningknowhow.com/plant-problems/pests/insects/spittlebug-control.htm
To be sure, you can dig up a sample, including grass and soil, and take it to the local Extension Service. This link will help you locate one: https://www.gardeningknowhow.com/extension-search/

If you're determined to have a St Augustine lawn, you'll need to consult a turf grass professional or the local Extension Service https://www.gardeningknowhow.com/extension-search/
However, alternatives to monoculture grass lawns are becoming more popular all the time: Here are a few articles to start you thinking: https://www.gardeningknowhow.com/lawn-care/lawn-substitutes/lagen/plant-alternatives-to-traditional-lawn-grass.htm
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