For the database performance tuning, it needs to run for some time (>=24h) not just 30 min.
The blog post you referred to, also says it got the suggestions from phpmyadmin based on their usage. Keep in mind that parameters can change a lot based on the use case (how many users, how much parallel usage, how much sharing, use of different type of app, …).
Can be, but you should be able to check the number of servers and children used. If you run close to the limits, then it should show in the logfiles of php-fpm as well.
That was considered less important by the developers because you usually visit the site very often so most of the static content is cached anyway. They tried to reduce the number of js/css/… files by combining an minimizing these files but because each app can use different js/css/… and that didn’t work well and they removed it.
When you navigate within the interface, you should see in the network analysis that a lot of content is cached.
You have set up system cron-jobs I suppose…