1) The Engagement Time is the time to be contemplating and sorting out long haul compatability issues like these.
2) Sit them down, tell them you expect to set an appointment for serious relationship talks with a plan for conflict resolution.
What is a good calendar date for them? And clear 2 hours to have this talk in. (Some things take longer, but can always make another appt. Don't try to go and go and go. That's exhausting for all.
3) Agree to make an agenda for the talk time so people have time to gatrher their thoughts. Not be all "deer in the headlights" when the appointment time arrives. Then you all can stay on topic and do not devolve into petty side arguments. If everyone knows their turn is coming for their bullet point things, they may be more willing to give the other persons their "air time" too when going through their bullet point things.
4) Include how to cope with emotional flooding and calling "time out" if needed.
5) Is everyone willing to own a piece of the elephant
to move the thing forward?
In service to the greater polyship? (Versus individual axes to grind?)
See if they are willing to attend a conflict resolution thing like that -- one that tries to be constructive rather then destructive.
Even if it turns out that the best thing for ALL people (versus best thing for individuals) at the end of engagement is NOT to be married, break up, or whatever other option... could all agree to just get through the process sanely ONE STEP AT A TIME first.