You have the option to make (non-computed) column User Specific when they are created:
No, just a single helper table with a single row, for all users. That’s how User Specific columns work. They allow different users to store different values in the same cell. Each user only sees their own value.
They still need to be cleared. Best practice is to clear them as the form is being opened. If you take a look at my Custom Forms Concept App, you will see how this works.
Yes, correct.
With a use of a Button and Add Row action.
Again, take a look at the link I gave you above. Make a copy of the template and study it. Pay particular attention to how the form is opened.