There are lots of online tools avaialble to find out the traffic of competitors like ahrefs, similarweb, seoviewtools, semrush, etc

it depends on the amount of traffic and new users visiting your website. the position of websites changes daily according to google. the factors which may affect position are thin content available on the website, keyword stuffing, etc

Yes you can do that but it will affect the over all structure or the design inside the container.
Containers have a 15-pixel padding by default, Rows and columns will be added inside the containers. i would suggest do not nest a container within a container.

There's not a universal best practice for doing it. Generally for large projects, it is recommended to separate css among multiple files for debugging and maintenance during development.