The naming guideline used in the widgets is not completely following the overall openHAB naming guideline. So one of the next versions might include breaking changes on that behalf.
You cannot simply rename Items, better delete and recreate them.
Yes, but every widget documentation has it’s own items section you can check.
There is no option in the UI, as this is not a simple task. It is safer to unlink and remove the items and recreate them.
Using “Create Equipment from Thing” will take the Thing name with added channel name as the Item name, so it is not absolutely unimportant.