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While I am uncertain of the type of bush you are referring, most blooming issues are the result of too much nitrogen in the soil (resulting in lush green growth) and not enough phosphorus (which is necessary for flowering/fruiting). One way to remedy this is to use a phosphorus-rich fertilizer or add bone meal to the soil. Here is more information: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/phosphorus-plant-growth.htm, https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/bone-meal-fertilizer.htm

For the most part, yes. As long as there is some remnance of a stem and/or root, the plant will usually regrow. If you're looking to rid the area of the plant completely, you'll have to make sure that you remove all of the root system.

Here is an article or two that you may find helpful: https://www.gardeningknowhow.com/plant-problems/environmental/tips-for-saving-cold-damaged-plants.htm, https://www.gardeningknowhow.com/plant-problems/environmental/how-to-tell-if-a-plant-is-dead-and-how-to-recover-an-almost-dead-plant.htm

The information in this article should help and works much the same for large shrubs as it does tree stumps: https://www.gardeningknowhow.com/ornamental/trees/tgen/tree-stump-removal.htm

These could be anything from grubs to whitefly larvae, or even garden symphylans (which is generally more common). This article will help determine if these worms are the ones in question: https://www.gardeningknowhow.com/plant-problems/pests/garden-symphylan-small-white-worm-like-bugs-in-the-soil.htm

I would actually recommend that you have your soil tested. This would be the best way to find out for certain what, if any, nutrients might be missing that could be causing the problem. Then you can find a suitable fertilizer based on that.