Lance has started 4 games, Rush has started 6.
Rush has 7 years experience in the NFL, Lance has 2. I don't think it's a stretch to think that Lance may get better with more experience. The Cowboys know what they have in Rush, he's a dependable backup for Dak. Rush knows he's not starting so you don't have to worry about a QB waiting for his chance. It's the perfect place for a QB of Rush's nature. For a guy like Lance who needs more chances to see what he can do, that's not a good spot for him. The fact that Lance is still on the active roster is proof the Cowboys haven't given up on Lance, they just think Rush is the better choice for backing up Dak right now.
It doesn't make sense to trade for a backup that is unproven at best and a bust at worst. This is the equivalent of trading for Josh Rosen, which is fine if you're trying to tank (as MIA was when they did just that) but a waste of time otherwise. Unlike, say, Fields, Lance hasn't shown anything in the NFL that indicates a meaningful upside. If you were to ask me before the season who I would take over Trey Lance, Tyler Huntley would have literally have been one of my top suggestions. I'd sooner trade for Desmond Ridder or Sam Howell than Lance; at least they have a decent amount of starter experience.
Depends on what they want in a trade. I think a 5th round pick would be worth the chance. I don't know if they would take that now, they probably would have taken it before the season started, probably not now. I've seen enough of Huntley already. I'd rather be starting Thompson in all honesty and I'm assuming that he's not starting because he's still recovering from his rib injury, but maybe they think Huntley has played well enough to start him, personally I don't see it. He's had a couple of decent throws but he looks scared to throw the ball in my opinion, every throw it looks like he'd like to pull it down and run.