There are two seperate accounts of space trains in the shows, though they are a bit more like space subways.
Does that really matter, though? It's unique, it's new, it's something the fans could love, and makes for a cooler scene (as well as more sense) than just a big ship carrying around deadly materials that could easily kill everyone on it should it be jostled in any kind of way.
Sure, but it's not called "Jedi Wars." This is a matter of personal preference, but I get tired of the Jedi every once in a while. It's about Han, I'd be incredibly upset if he met a Jedi. And, as I've said before, there was really only one heist scene.
What the critics of TLJ didn't like can't really be fixed by IX, excluding giving Snoke a backstory, or maybe explaining hyperspace ramming a bit more.