If I were you I would begin noting all the things you've mentioned in a diary that you buy especially to log this stuff.
Threats to kill your new partner are serious and personally I would be wanting to put a stop to this and get police involved as that is a crime.
I'd then be making her available for contact, dressed, bag packed, take a picture with a timestamp then if she refused not force her, let dad know, and leave it at that.
I'd cease communication with dad that wasn't practical, so only responding directly about contact.
I'd let him go to court and I'd then use the log to form my position statement linked to the Welfare Checklist which would include alienation of your step parent, assault (threats to kill), arrests, and the fact you had made her available every single time.
You aren't obligated to make a child go to contact. You are obligated to make them available and encourage them to go.
A court process would not be nice but provided you knew how to navigate it you wouldn't come off too badly.
Does he have the time, inclination, and money to apply to and follow through on court? Would be self-represent or would he be hiring solicitors?