hmm
i rescued a very distressed siamese cat a few years ago, and she had a problem that sounds a lot like missy's.
do you know anything about her previous living situation? if she was mistreated or out on her own, she may very well just be stressed out or afraid of running out of food.
in my siamese's case, she was mistreated/neglected. she was one messed up cat. she'd gorge herself, barf a couple times, and go right back to the bowl and gorge herself again. it was a vicious cycle. she would growl and hiss at the other cats if they came near her while she was eating, and she'd scream like a child if i didn't leave food out. i started to watch her eat, so i could pick her up and love on her when it was obvious she'd had enough. i started buying food with no artificial stuff in it too, that seemed to help a bit. she was still weird about food though. you couldn't look away from yr plate for a second without her hopping up and running off with yr dinner. she ripped the lining out of my boxspring, so she'd run in there and hide with her food loot. after several months and endless patience, the gorging slowed to a hault. she never stopped stealing our "people food" but at least she wasn't gorging and purging. i was extremely allergic to her and had to find her a new home eventually. she was really friendly, when i'd have parties or just have people over, she'd go around the room from lap to lap and just love on everybody. she found herself a nice new home in no time. sadly, a few months later, her stomach turned over and she had to have surgery. i'm not even sure what that means...the new owner had moved away and i heard it from a mutual friend. i do wonder if her stomach illness was caused by the gorging/barfing though, or vice versa. i sure hope she's ok.
it seems kind of awful, but you might want to try letting missy have it her way for a while longer and see if she gets the picture. i would think that scheduled feedings would just make her hunger more intense and frantic about when she's going to get her next meal. you might also try giving her treats or something in between feedings to tide her over if scheduled feedings are still necessary. its a hard situation to conquer, fer sure. good luck!