- the South was Reagan's weakest region.
Yep, it was tough for Reagan to defeat an ardent Christian Southerner. But he pulled it out.
- Reagan had just been nominated and, when planning his campaign's schedule, the best fit for an important event was the Neshoba County fair.
Yep, probably true. And even if it wasn't he certainly had every right to campaign there.
- Reagan probably didn't even know he was going to Philadelphia until he got there (candidates don't plan their own events generally), and even then it's doubtful he recalled the killings many years prior.
Candidates don't plan their own events but they certainly are informed about them. I also find it hard to believe that Reagan would forget about an event which resulted in the FBI sending an army of agents to Mississippi only 16 years after it happened. 1989 is not some distant memory to me right now, why would 1964 be a distant memory then?