Can you see the texture when you place it in the world? If so, you aren't registering the item model. If you can't place it in the world, that's an FML registry bug and you should look at the RegistryHelper in ironchest's 1.8 branch.
You will probably be able to add tabs again when forge (the api) is released. As for your blocks not working, post a screenshot of your file hierarchy.
AT's and coremods are different things. AT's can be used to change the visibility of a field. Byte code modification is used to change vanilla behavior in ways that forge does not have events and hooks for.
Of course you need a main class. How else do you plan to get forge to see it? Call the method to run you version checker in the ClientProxy class, and register the proxy in the main class. Follow a modding tutorial.