1.

Q: When should a participant initiate a termination protocol in
interactive 2PC?


A: When CANCEL message has been sent and the coordinator hasn't responded in 
reasonable time (the interactive 2PC has timeouts for coordinator failure).

Sending the CANCEL message isn't part of the termination protocol, as a 
participant can decide to try to abort after voting YES and it is normal 
behaviour in I2PC. Note, that even if a participant has sent CANCEL, it might 
have to commit the transaction. This happens when the coordinator has already 
reached a decision, but the participant hasn't received the decision yet. So 
the participant can not abort right after sending a CANCEL message.