Hi,
Configuration 3 is okay. FTP access has no influence on anything. Regarding site visitors,
SEO and AdSense, everything is fine, if you make sure that any hit on
www.kjellbleivik.com/subfolder/ are automatically redirected (301 redirect) to
www.digitalstart.net/.
Configuration 2 is okay too. If a site is not publically available, it will not come up in search engines and I guess that there are no AdSense ads on this site anyway. Use the 301 redirect as explained above if there is any chance to access the pages of the add-on domain through the main domain.
No reason to get penalized for these configurations. Why would you be penalized for not being evil ? I use similar configurations for years.
Jean-Luc