Geez;
Lots of questions there.
Lets get to the common denominator first.
All engines fail. Means that. Who cares how, as long as end result is the same.
I guess the case could be made.. engine still attached to mounts or not (CofG and drag issues); maybe rotating or not (hydraulics and drag/fire issues). I think the intent is...
Is the loss of all engine power going to affect you being able to control the aircraft for the remainder of flight (until altitude, airspeed and ideas run out). Nothing in regs says where that happens (unless you wanted to discuss ETOPS and a REALLY bad day). Case could be made that APU and RAT requirements extend that time a little longer.
Seems straightforward all of a sudden. Some method to do just that must be available.
Then comes the pudding (as in proof's in the ....).
Demonstrate it. Not a big prob, actually. Production test flight usually accomplishes this.
Analysis is demonstration on paper. (acceptable to certification authority). Have no idea about JAR.
HTH.