A sign by your pottery explaining your policies could be your answer. Short and to the point. When you have this type of customer again just point to the sign and say our policy is..... As for the switching up and changing her mind,,,,,,,,I've dealt with this type of customer before and after awhile I tell them nicely I'll let you look around and let me know when your done. Re-writing the receipt.... Probably a good thing for you so she doesn't come back and accuse you of changing it. Her nastiness,,,,, I think you handled that well.
For every customer like this I hope you have a hundred that are appreciative of your work.
I know I have had customers in the store that have adult 'ADD' and act this same way with the flitting and running around and don't let you talk ,,,,that doesn't excuse the behaviour though.
The sign should say in bold letters;"No crackpots allowed. Thank-you."