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I find that as they age in my Texas garden, they do not look as they once used to look like so I prune regularly but have not replaced them yet. I do get cuttings and plant them elsewhere too or give it to friends. I do not make hedges with them though.

Fish will nibble on roots out of curiosity. This plant has been shown to be toxic to many living organism, but little seems to be known about the toxicity to fish. I would not recommend planting them with any fish that you would not want to lose.

It could be a moisture issue (too much or too little) or a light issue. Here are its growing requirements:

https://www.gardeningknowhow.com/edible/herbs/rosemary/rosemary-grown-in-containers.htm

Yes, it should regrow. Just keep it watered as you normally do.

Rosemary dries out quickly without proper light and humidity. It probably suffered some transplant shock in the transition to pot culture. Is it still outside or did you bring it indoors? They are difficult to grow indoors. Dry, winter heat and drafts can be lethal.

I'm not sure how you want to use the leaves but if it dried out on the plant, that's not a good sign. Make sure it is not overwatered or underwatered and it has plenty of light.

Here is how to care for it:

https://www.gardeningknowhow.com/edible/herbs/rosemary/rosemary-grown-in-containers.htm