1. I would suggest grabbing an ePub and converting it to kePub to see which renderer you prefer. Calibre with the extended kobo plugin will do this for you automatically, but I think that it's just changing the extension to .kepub.

2. The only way to alter the firmware is through updates and hacks. Anything else that you do can be undone by removing the .kobo directory. This will force the .kobo directory to be rebuilt, including the infamous database. If you're using sideloaded ePubs, you don't even have to worry about replacing your library since that is stored outside of the .kobo directory.

Even if you decide to use hacks, a factory reset will reset the entire firmware to its factory state. (A copy of the original firmware is stored in a separate partition. This partition overwrites the system partition during a factory reset.)

3. This depends upon what you call advertising. Kobo does have a bookstore and recommendations that are accessible from the home screen. These are easy to ignore. On the Glo and Aura, the recommendations tile can be dismissed by long pressing on the tile and deleting it. That is as intrusive as the advertising gets, and some people regard this as a feature. As far as I recall, the recommendations tile pops up even if you falsify the registration information (i.e. alter the database, without creating a Kobo account) but it won't pop up if you never connect to a WiFi network.

I hope that answers some of your questions.