A message forum for general discussion. Please come and chat with others!
1. As far as taxes go, you need to seek advice from a tax lawyer or accountant. Generally though, Amazon have no idea what your income situation is, so pay you the dues for any sales and the onus is on you to make declarations as necessary on your tax returns.
2. Generally, you must declare income in the tax year you receive said income payments. Speak to a tax lawyer or accountant.
3. You open an account on Amazon using your real name, and each book is published under whatever name you wish.
There are many cover artists working on commission that you can find either in facebook writers groups or deviant art.
Finally, I would add that whether you self-publish or go the traditional route, marketing your work is your responsibility, not an agent's or a publisher's (although both might give advice and may do a small amount of marketing themselves). Note that any marketing they do (if they do) is unlikely to generate more than a handful of sales. It really is your job.