OP I think what most people are concerned about is that he doesn't have a stable gradient set up, and seems like he doesn't have the hotspot that can facilitate a feeding response and digestion. Unless you're constantly monitoring his cage to move him to cooler/hotter spots as required, his temps are going through wild fluctuations.

And it's totally your decision to return an animal you're unhappy with, but keep in mind that most people don't even offer food to a new snake until after a week has passed. Not eating for 7 days is by no means a food strike.

With regards to the size of prey your feeding, it looks appropriate, but it's hard to tell without any references. Could you possibly post a pic of it next to the snake? And is it a mouse or a rat? Also, please do not release it into the wild. Could you perhaps rehome it or return the feeder as well?