How do ADS-B and ADS-C get used on transoceanic flights?
1) ADS-B will stop working when the transponder is switched off at the gate after arrival in Tokyo. Airliners use the 1090 MHz Extended Squitter variety of ADS-B which is the worldwide standard.

2) I don't have hard numbers but I guess it is approximately 90 percent of the flights. At most economical cruise flight levels, ADS-C is mandatory.

3) ADS-C is a common system. For aircraft not equipped, position reporting of HF is the alternative. And soon space based ADS-B will be available as well.