A good place to start is the sample application provided on the developer docs.
- Guidelines would be to just make it as usable as possible. Take a look at the others available on the market to see what you should be aiming for
- Yes, services can do most things, including internet; provided you have asked for those permissions
- You can open activities and do anything you like n those if you run into a problem with doing some things in the keyboard. For example HTC's keyboard has a button to open the settings activity, and another to open a dialog to change languages.
Take a look at other IME's to see what you should be aiming for. Some (like the official one) are open source.
