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You can dig it up and put it in a pot to bring indoors for the winter. Cut it back some so it's not so hard to handle. Make sure you get most of the rootball. Water well and let it acclimate for about a week before bringing in. Then site it in a south or southeast facing window. Water only as needed.

Here is more info on popcorn cassia:
https://www.gardeningknowhow.com/ornamental/flowers/popcorn-cassia/popcorn-cassia-information.htm

That is a bacterial or fungal blight. Common this time of year, but with some perseverance and time, you can get rid of it.

Your first line of defense will be to clean up any debris around the tree. Then trim off as much of the affected growth as possible. Treat with a fungicide/bactericide, and apply DOLOMITIC LIME and WETTABLE SULFUR, according to all packaging instructions, to the soil around the tree.

This should suffice for treatment, but you may need to do it for a few seasons until it is cleared.

These articles will help: https://www.gardeningknowhow.com/garden-how-to/info/applying-bactericide-to-plants.htm

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

https://www.gardeningknowhow.com/plant-problems/disease/black-spot-fungus.htm

This collection of articles will help you with the care of the tree: https://www.gardeningknowhow.com/category/ornamental/trees/cassia

More than likely. This tree has quite the root system, and will easily clog septic lines and systems.

I would contact your local extension service for help in this situation:

https://www.gardeningknowhow.com/extension-search

There is no cut and dried answer to this question. You need to contact your local county extension service about how these might affect the water system there. The trees and their roots will grow according to weather, soil conditions, pruning, feeding, and other specific conditions. Here's a Florida article about cassia; it does not mention intrusive root problems: https://hort.ifas.ufl.edu/database/documents/pdf/tree_fact_sheets/casfisa.pdf

Here's an article about how to prune them: https://www.gardeningknowhow.com/ornamental/trees/cassia/cassia-tree-pruning.htm

We don't sell plants or merchandise here, only free gardening advice. If your local nurseries don't carry the plant, have you tried online nurseries (besides us?)?

Your soil may need more iron. The tree likes a balanced pH, so you may want to do a soils test and see if there's something like iron that's lacking in the soil. Other possibilities are more serious, so let's wait to see if amending your soil will help.Read these articles for additional info:

https://www.gardeningknowhow.com/plant-problems/environmental/leaf-chlorosis-and-iron.htm
https://www.gardeningknowhow.com/ornamental/trees/cassia/growing-cassia-trees.htm