A while back, my Sim got with Don L (as we know, he has the noncommittal trait) and romance bar was full but friend bar was like half full? and she proposed and he rejected. I am surprised she didn't die from that! Don L. did marry another of my Sim but she got tired of him feeling trapped and always tense that she sent him on his way!
I think if you made it thru proposing, then hopefully it will go alright unless you call the wedding off?
If you try it and it fails, just don't save it.......but do come back and tell us about it, LOL
Edit: Oh my, I just re-read your post. You are looking to break them up before they make it to the alter :D
Carry on and do tell >:)