Yes! You can use the full screen mode to hide the rest of the manager when editing content.
In your configuration set, under Toolbar, add
fullscreen to your toolbar buttons. Save, and use the button whenever you want a bit more clarify.
To limit the width of the content to make sure it's easy to read, add a Maximum Width under the Appearance section of the configuration. A value around 800 or 1024 works nicely in most cases.
Most of it, yes.
The Redactor 3 upgrade will automatically create a configuration set based on your Redactor 2 system settings. That wont be enabled by default, but will keep most of your configuration in tact.
The upgrade does not cover context overrides, or overrides from the custom template variable. Those will need to be manually applied again after the upgrade.