I understand that this must be super frustrating, to say the least, for the OP, but I'm not sure the seller really merits being called a Bad Guy here.
It sounds like we know the snake was shipped, and we know the buyer did not receive it. OP, you've said it was either stolen or misdelivered.
So if stolen: It sounds like there wasn't that much time between when you got the email that it had been delivered and went to the porch? I know you've said no Fed Ex truck came by. Can you be 100% sure? Those guys pull up, rap on the door and are back in their truck leaving awfully fast. If you were in another room, or distracted, could you have missed hearing it? Unless you were literally glued the front porch, can you be absolutely sure? I am not calling you a liar in any way, shape or form. I believe you that you never saw/heard the delivery. But, is it possible it occurred? How quickly would it have to have been stolen? It seems like it would have to be pretty fast, but it could have happened.
Misdelivered: this seems more likely. It sounds like the driver had to come back by to be sure it was your place. The "I thought so" or whatever he said, sounds totally odd, but maybe he really thought he remembered you, or was just refreshing his memory to get his story straight. Still though, I assume the box was properly labelled? I'm just picturing your average person getting an unexpected package with a live snake in it. I don't know the probabilities, but most people would probably call fed ex in a hurry and/or their local news with a "live python left on grandma's porch" kind of story. In which case the package would be found and fed ex would know about it. A smaller percentage would look upon this as manna from heaven and have a nice new free pet. In which case the package is never seen again.
It's hard to say which occurred. I totally get that you are really frustrated. But look at it from the seller's point of view: he did his part, shipped the snake out. He did call Fed ex (several times per the text you posted) and they are telling him absolutely, it was delivered. So he is in the position of believing Fed Ex, or you. No offense to you, but he doesn't know you at all. If a convincing fed ex agent is telling him, "yes the driver confirms it was delivered, he knows the address, he's been there before" maybe he believed them over you? Maybe he thinks it was stolen off your porch?
It sounds like the seller did try to work with you at first, but it looks like from the text he posted, he considers the matter closed now. I do wish for your sake that he would file the claim to help you. But, I don't think he is not filing it out of malice or just general non-responsiveness. It sounds like he made his decision to go with what fedex is saying.
In terms of the shipping w/o signature required, that is pretty normal for reasons detailed before. It sounds like he could have made that clearer, but you also could have asked for clarification.
It's a totally sad situation and I do feel bad that you didn't end up with the snake. It just seems more unfortunate than malicious or "bad guy" scammer level.