"Pickups line the highway as the townsfolk come on over to observe Ray round off the pitcher's mound, pour the lines of lime, mount floodlights and bleachers."
In dictionaries, I could find "come on" and "come over to", but not "come on over to". What does "come on over to" mean?
Pretty much the same as 'come over to'.
"on" adds nothing at all to "come over to"?
Not really. It may perhaps sound a bit more folksy and informal.
No.
"come on over to" is equivalent to "come over to" in meaning, but the former is regional in usage?
If you'll note, he said it was "folksy." That's a great way to describe it.
It's not regional, it's not slang, and it's not wrong. However, it's not formal. It's the way "plain folks" talk to each other, in a friendly way.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
I think you need to ask a professor of a freshman English class.
In my opinion, when describing something as small-town as gathering to watch someone strip a baseball field, it makes perfect sense in the context. But then, thank the dear Lord, I do not have to grade freshman papers.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.