Actually, this grammar is CORRECT and happens in reported speech.. To make something reported speech (reporting someone else's words) you take the original sentence BACK one tense. Example: Robyn: "I AM coming over tonight" --> Robyn said she WAS coming over tonight (present continuous --> past continuous) Robyn: " I CAN come over tonight"---> Robyn said she COULD come over tonight (present modal ---> past modal) Robyn: "I CAME over last night"--> Robyn said she HAD COME over last night (simple past ---> past perfect)

Jack is probably right. "She said she's coming over tonight" or "was coming over tonight" are indistinguishable. It's probably just a regional thing. In the US native speakers wouldn't even notice the difference between both sentences nor care and would consider the two as essentially saying the exact same thing. These trivialities are not worth worrying about.

"She said she will be coming over tonight." would be the most correct way to say sentence #1. In the US it is extraordinarily common for people to say sentence #1 in the form that you wrote, even though it is not correct.

It's very simple: you report the facts as true at that time in the past. There's no need to think about the present in either case.