Hi chuckiii3,
1) No, it is not possible to offer customers a choice between flat rate and calculated shipping when listing through Auctiva.
2) To offer non-Auctiva shipping insurance, please didable the Auctiva Shipping Insurance setting within your account at Auctiva -> My Account(Tab) -> Auctiva Shipping Insurance Settings. Once you have disabled this setting, you will have the option to offer other types of shipping insurance when creating your listings.
-Mike