Ribbon Workbench - Is the button added at Entity level or Form level?

 Hello All,

Q.1 I want to know if the button gets added at Entity level or Form level by Ribbon Workbench?

Q.2 I have added set of buttons in one solution and another set of buttons in another solution by another publisher. If I uninstall one solution should the buttons included in the solution gets removed? should it also get buttons from another solution get removed?

Q. 3 I have a CRM environment of a client, where sometimes our buttons added by our unmanaged solutions gets removed (though other customization stays) and none of us know the reason.

Can anyone guide how can it be possible?