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1. No. You tell phpmyadmin your MySQL username and password; phpmyadmin looks up your privileges via MySQL. You cannot override those privileges unless you have administrative (root) access to MySQL.
2. XOOPS stores almost everything in the MySQL database. Exceptions would be the configuration settings in mainfile.php, and uploaded files. Of course third-party modules could be written to do things differently.
3. I don't know.
4. No. You need administrative access to the web server to do that.
I'd recommend getting a commercial hosting account with a provider that offers a range of hosting plans, with no extra fee for upgrading to a different plan. Then you could start with a small, inexpensive plan, and upgrade later if needed.
You could also install XOOPS on your own computer, using something like
XAMPP, to get an idea of the resources needed.