If you don't give them anything but the starter grower crumbles then they will eat it and that is what they should be eating at this age. You are giving them way to much scratch and then they are filling up on bugs and foraged stuff so they aren't hungry when it comes time to eat what they need. At between 11 and 15 weeks they aren't very big and won't need a lot, they still have tiny tummies. Nix the treats and make sure they eat what they need. Its just like taking care of children if you fill them up on sugar they won't eat their veggies, take away the suggery treats and the veggies will start to fill them up slowly.
A 50 lb bag of starter grower for only six chicks should last a couple months anyway. Personally I doubt seriously that your feed has molded as long as you have been keeping it in a dry environment. I use a plastic storage tote and just leave the feed in the bag, put the whole thing in the plastic tote and close the lid after you've filled the feeder.
The only difference in crumbles and pellets is that pellets are pressed formed crumbles. The ingredients are the same, crumbles are designed for large poultry operations with screw type feed delivery systems, Pellets are generally for the small scale farmer. EXCEPT, my local feed store only offers the starter grower in crumbles because the mill they use doesn't make starter grower pellets, they do offer layer feed in both crumbles and pellets. I wish they offered starter grower in pellets because it feeds better in the small feeders but most baby birds like the smaller crumbles as its hard for them to eat the larger pellets.