Q1

2 and 3. Pretty permalinks are user-friendly and will benefit SEO

Q2

4. The sidebar is the most common place for widget areas and the footer is the second most common. But there's nothing to stop you adding them anywhere you like. They can be very useful above and below the content, for example.

Q3

1. To add a menu, you need to follow three steps: register it in functions.php, add it to header.php (or whichever template file you want it in), and then set it up in the Menus admin screen.

Q4

3. archive.php from the parent theme, as a category listing is an archive page. If the child theme had an archive.php or category.php file, they would override the file in the parent theme.