Well I'd say they want to be sure they have the real person who owns the accounts in front of them and they can't be sure. The CPanel usually provides you with access to databases and all the domain settings. So if you want to do bad this is the absolute best way to go to get sites down usually.

As far as I can tell from checking this snippet it is NOT redirecting traffic to your index.php file and public/tmp/ and a couple of more directories. I'm not that familiar with Apache environments, somehow more Nginx so I can't help you with the other three lines.

Yeps we do the same thing, they won't have access anymore in our CMS so we'll include on their author bio that the editor has left the company and what company they went to work for. In some cases in their new job they also want to contribute and that leaves their (old) content still as it was.

I would say Yes, if you make sure you're redirecting the right pages to the working URLS on the other site you should be fine. Also make sure that sitemaps are updated as Google won't like redirects within a sitemap.