Many balls are alot more hesitant to switch from mice to rats than yours seems to be. NEVER OFFER IT A MOUSE AGAIN! As for the shedding, I assume you mean it did shed, right? I think you just missed the "blue" period and/or pink belly. Sometimes it can sneak by you, or you could be looking at a snake in blue and just not notice the color difference.
Don't worry about leaving a bit of a lump after feeding, especially in a baby ball. I find with balls that you can stuff them if you try, but when they are done eating for a while they are DONE! So feed 'em while you can. (But, of course, IMHO overfeeding is cruel. 5-7 days is my personal max of offering prey for a young BP. This length increases with age and size.)
Whatever you do, make sure you are leaving enough time between each prey item for your snake to properly digest them. For example, now that your snake has eaten a larger rat and you see a lump, wait for it to go away before offering the next prey item, or even watch for your snake to defecate.
Right now I have four balls, one is a hypo and one a pastel, two normals; and with my limited knowledge I believe that with time you will know when they are displaying hungry behavior, and when they are not interested. With new BPs, the biggest tricjk is to make them feel safe and get 'em started. I count you very very lucky to get your little guy on rats so easily.
Just remember, the number one rule with ball python feeding is patience-- don't freak when it does not eat; rejoice when it does!!! Good luck!!