I still think E and T make more sense. His loneliness might be more correlated with being socially awkward rather than actually disliking people, and he created Hobbes to compensate. I is possible, but not likely.
F is... stupid. He is not warm at all toward his family, and most of the time he's just friendly rather than really warm towards Hobbes. Daydreaming and acting on what he values as important are NP traits rather than F traits, and NTPs are just as likely to do so as NFPs. He is not, and I repeat, he is not good-hearted at all. If he were to be an F (which he is not), he'd be an extremely selfish, spoiled and morbid Fi. Good-hearted people do not get kicks from smacking the neighbor upside the head with a snowball, or depict snowmen dying a gruesome death. Furthermore, if he was F he'd be much more motivated by Moe's bullying and much less so by boredom, whereas in reality he doesn't give much of a shit about Moe. To cinch the deal, his interests just scream Ne Ti on crack. They include:
-Exploring the forest
-Making up games on the fly
-Building gruesome snowmen just to creep out the neighbors
-Building superfortresses of his own design
Which leads me to conclude he is, in fact, an ENTP.
Hobbes is a rather caustic and diligent character -- His role is to keep Calvin grounded, or at least do his best. He does however have a significantly evil streak, which eliminates INFJ (or non-sociopathic INFJ at least) as a possibility. He's not S either, being that he's every bit as creative as Calvin, although more deliberate than him (J). He almost always wins Calvin's games through dirty trickery. He further is incapable of doing things that involve minute detail, like basic math (7 + 3 = 73 for instance).
On the other characters, you're pretty much entirely off-base. Susie is creative, bright and grumpy, and doesn't seem to have friends or like people. She plays a similar role to Hobbes, but without his naughtiness. Furthermore, she is at most an ISFJ, given she is too domineering and goody-goody to be any sort of P.
Dad is an ENTP who happens to superficially look like an ISTJ, to which the OP is testament. ISTJs do not fuck with people like that. They just don't.
Mom could be ISTJ, but she seems a little short-tempered (and warm when Calving isn't pissing her off) for it.