The problem with documentation is always that you need: a) a lot of time b) to be able to look at it as an outsider
We used to have someone in charge of documentation, but he left the team. The two main developers both have (had) health issues, so the limited time they had available wasn't spend on improving documentation.
Documentation that, as you rightly said, lacks support for beginners. This has a reason, the framework founder didn't want to provide support to beginners, and made a conscious effort to deter them by not proving this documentation, something that we have struggled with ever since.
The DB documentation has my attention, I've started rewriting them to provide better support for specific RDBMS platforms, such as Postgresql. I'll keep your remarks in mind when I do.
Having said that, please ask your questions here if you have them, or contact one of us directly on IRC (#fuelphp on freenode).
As for classes, you don't access the the core classes directly. All core classes are aliased to the global namespace, so you can extend any core class in your app. So don't use
use \Fuel\Core\DB;
You can just use DB when your class isn't namespaced, or \DB ( or use DB; ) when your class is.
When it comes to Models there are three options: - use ORM - use Model Crud (which uses direct db calls) - code your own model
The first two are supported by scaffolding, you can use a commandline switch to indicate which one you want to use.
If you want to code your own model, you'll have to look in the scaffolding code which Model methods are called ( won't be that many, stuff like get() and save() ) and how properties are set. Make sure your models are API compatible with that, and you should be able to slot your own models in without any problems.
I'll have to think about your suggestion. It is indeeed assumed that you know object oriented PHP. Perhaps add a "pre-requisite" page with some guidelines to knowledge required, and info on where to find that?