Hi,
1) ADS-C is part of CPDLC system when user logon to the AFN. If GPS is down, as long as there's VHF coverage, it will be a means of transmission. Referencing to MEL item 23-15-01
2) I'm not sure. Manuals don't talk much about ADS-C.
I would think ADS-B and FMS are independent but need confirmation