jangus
Hi
Let me reply it one by one.
1. JA Sensie is a Joomla template that comes with Guru style support, if you will not use this template Guru use its own style, and also make sure the template has support for Guru like any Joomla template support.
2. Any user who register on site goes to Joomla DB as core, this is normal. Guru has own DB for student and teacher profile.
3. To make the site like JA Sensie on a 3rd party template its a custom work job and not related to Guru too as its template own style part.
4. It not only custom css in JA Sensie, it also has override for Guru files in template html folder.
I do not understand this part as its same questions. If you use JA Sensie as default template it will work as in the demo site for Guru pages.