1. To get fat Ronda:
Go through her first two stages to get her the bulking powder and protein shakes from the Health Bar at Eve's Place.
Instead of getting the steroid from Eve's Place, go to the lab and get the muscle enhancer from Tiffany. (If Tiffany's been immobilized or eaten, you can still find it in the lab.) Then give that to Ronda. Next time you see her, her muscle got turned into fat.
2. Not aware that there's a way to give the cheesecake to Nicole instead of Lily that I've seen.
3. Heather, the food truck owner... just keep going to the park (purely random chance if she'll be there, no help for it), the more you buy from her the more weight she'll gain. Eventually there will be a chance you'll encounter her food truck on the way to work. When she hits nearly max weight will be an option to give her the sandwich to get her to max weight.