I've been pulling my hair out with this mission, glad I found this message board to see that I'm not doing anything wrong. Last night, I tried killing just 3 of the flankers (using TWS) and letting the fourth guy escape, but I still got the ROE message. I never told my wingman to engage anything, so he was pretty much just acting as a meat shield, to ensure that he didn't kill that last flanker. I saved the .trk file and the debrief, if @feefifofum would like to see them.
I've been loving the campaign, I wish there were more like it!