After Frank loaded the Desert Eagle's magazine, he should have chambered a round by pulling the slide back and releasing. This action would automatically cock the hammer making it ready to fire. In the beginning, he cocks the hammer, making a noise. It is plausible that he released the hammer safely in the time it took for Jim to get to the back door, it is unlikely.
Fentanyl is an extremely powerful painkiller. Even if you are addicted it takes several days to experience withdrawal symptoms, especially when you are abusing it by patch. Jaclyn could have gone as long as a week without using a patch and might have overdosed had she used too many in too short a period of time.
Anna was on foot when she left her house. Given the fact that there is (according to numerous characters) "only one road out of town" finding her should have taken minutes, not the several hours that it did.