Newt, once coverage has been extended by the carrier, the nonwaiver or reservation of rights 'dies' (whether it is null and void is a legal question beyond my background) as the purpose of both is to allow investigation of the claim with questionable coverage until the coverage question is resolved and coverage established or denied.
Newt, I can't envision a scenario where it would become a two edged sword after the claim is closed and paid.
Newt, please feel free to send me any other questions which you have directly to my email address CADOAIC@aol.com