1) ADS-B is a transponder feature that takes its position input directly from the MMRs - ADS-C uses the FMC computed position, thus a GPS failure will not affect it directly - the inertial position will still be available, possibly even a composite position through radio updating. However, VHF coverage is of course not a requirement either - it uses any available data connection, be it VHF, SATCOM or HFDL.
2) I would assume a dual FMC failure means you won't be able to use ADS-C anymore, the MEL is unclear about it because FMC inop dispatch is not permitted at all (at least according to the one I have on hand) - ADS-B is unaffected.