Is it possible to have two templates in one place, one for the public and antoher one for registered users?. If so, how could I?. (Situation: Public view with 3 columns, register view with only 2 columns - Kunena requires wide space!).
Probably best approach is using Joomlas built in feature to assign a template to a menu link. You could assign an alternate template to Kunena for example. Aside from that no idea.
I read the article on subdomains for each user, but I'm not clear if in this way is partially or completely can customize his/her profile, like a personal website (for example, using another template).
CB does not provide this. It's profiles within your site and does not dynamically generate subdomains or anything of the sort.