Yes you kind of "own" your ebook, but.... The company that sold it to you can remove it legally form their listings, taking away your ability to download your book.
Yes, but computers are prone to failure, you are depending on a device to hold all your media. I've had backups and backups of backups, still losing things to failure. My books take up space, but they will be there unless my home burns down.
Not necessarily, you can self-host. I also listed it as a backup, for actual storage on thumb drives, SD cards, etc. It's really not difficult, files are far more secure and safe than a physical book, which itself degrades upon use.
You have to pay to self host, you have to pay for cloud storage. I am not going to pay to keep books, I buy books and put them on a shelf. I can pull one off and read it when ever I please, I don't depend on any devices to read them, and I like it that way. This is how I feel.