Hmmm...everything I've heard about treating is that treats are great as long as they are the right kind. Green scraps and fruit scraps and nuts/seeds, fully cooked beans mealworms cheerios and catfood are all good treats and can even benefit layers if there is natural calcium present like yogurt and extra protien like catfood and worms. Corn, rice, cooked potatoes, starches and oils or salty things like chips are the ones you want to stay away from. The ducks love cheerios and mealworms in milk. Dabbling for their food is their favorite way to eat.Have you tried checking their vents? The difference between a laying hen's vent and a non-laying hen's vent is big. A non-layer will be quite small, dry, and almost puckered; basically no different than a chick's vent. A laying hen's vent will be much wider and more open, and very moist or even wet/drippy. You can definitely tell who is laying by this method. Also, chickens have 2 bones on either side of their vent area; if you hold your chicken from below and slide your fingers back until you feel those bones, see how many fingers you can comfortably rest in between them (on the outside of the chicken of course, do not stick your fingers in their vents). If it's just 1-2 fingers wide they are likely not ready to lay; if you can fit 2-4 fingers between those bones GENTLY, they've probably passed eggs already, or are getting close. HTH!
One thing I get from reading your post is that your chickens may actually be TOO spoiled, unfortunately. We all like to spoil our chookies, but mine were infinitely healthier and started laying quickly when I stopped giving them so many kitchen scraps/treats and kept them solely on their feed and free range. I highly recommend removing all treats (with the exception of a small handful of mealworms each day, maybe) and upping their protein. My 9 big girls all started laying between 19.5-23 weeks, very shortly after I upped their protein and stopped treats. HTH!