This plugin allows you to insert code into the template head without having to edit the template files directly.
Yes, but you may easily get confused and mess up your whole site. It is better to learn the code first before you use this plugin.
Disable the plugin. If your site is still messed up, then there's some other problem.
The problem lies in whatever you typed/pasted into this plugin's option screen. If you're really stuck, then try clearing out the code and starting over.
That data is stored in your database. If you made a backup before you deleted the data, then you can restore it that way, otherwise it is gone.
Right now you cannot. If there is sufficient demand, then we'll add it.