Are there any official best pratices for developing EPiServer CMS solutions defined anywhere?
I know this doesn't answer your question for the CMS-solution but on the current roadshow they did a demo for an EPiServer ECommerce "Reference" site that they will be released in a month or two. They were honest saying that the other Template-sites that they have out now aren't any "Reference"/"Best practice" sites. This new Reference site will first be released on Github and thereafter as a template in the EPiServer extension for Visual Studio, just like the Alloy template (but only for MVC).
Mayby someone else can fill in if I got anything wrong.
As far as I know - not in written form yet. Any particular topic you are interested in?
Frederik Vig's old "Site From Scratch" series was a great introduction:http://www.frederikvig.com/2009/12/introduction-create-an-episerver-site-from-scratch/
Someone should write an official guide in that manor for EPi8.
Wow, rekindle an old post for shameless plug, nice