This one is always tough! We had this problem with my youngest daughter as well.
I do agree with you husband though. Once this stock of diapers is gone, don't buy anymore. Keep reminding her that once they are gone, they are gone, and she is going to have to start going in the potty. I'm sure there will be accidents and tense moments, but she will get the hang of it. Be ready to through away some panties! You may reward her with new pretty panties if she starts going on the potty. That worked for my girlfriend.
My daughter went the constipation route. She would with-hold her stool for DAYS!!!(diaper or no diaper) She would start doubling over, crying, pacing, it was awful. My pediatrician recommended Mineral oil in her milk a few time a day to lubricate, we did enemas when it got really bad and laxatives that made things worse! We finally went to miralax to help her go. But I have to say what really worked was the lots of patience and chocolate! I know you are getting conflicting information here (the M&M's) but it was around Easter and they had those little chocolate bunnies, so every time she went poop on the potty, she got a "poo-poo bunny" Sounds strange(and gross), but it worked. After a few bunnies, she never had another problem with constipation or going potty. She did want to keep getting chocolate for going potty for a while but after the box was gone, I told her we didn't have any more, she was fine with that, the rewarding stopped, and the pooping continued. Her eating habits are very healthy and she does not continue to ask for candy and chocolate every day.
Hang in there!