Not Crossing of the arms for the round off: A common problem. Most of us will instruct to follow a line on the floor as a target to correct form.
I think a better suggestion in addition to the line target would be to step farther and lunge deeper into the round off. And the best way to correct the habit is to get back into the cartwheel skill development. Most likely, if they are not reaching their far arm over the other arm, their cartwheel is not effective, anyway.
This instruction and exercise to go back to a more basic skill will most likely be criticized by other coaches and other onlookers, unfortunately. Because they’re stuck in moving forward and advancing in levels of skill. Then they also get stuck in non stop verbal corrective statements like “reach over more”, “get your hands on the line.” Or worse, a focus on negative, sarcastic suggestions: “stop doing front handsprings for a round off.”
Think how the error that you’re directly looking at is preceded by other movements. In the case of the not-reaching-of the far-arm for the round off, what do you think is happening that is really causing that? I believe it’s the depth of lunge, the closed shoulder angle, the non stretched body alignment on the hurdle, the timing of reaching of arms up along with the hurdle and reaching down, and many other factors contributing to the directly observable technical dysfunction.
go back further in the chain of movements. There, you’ll find the answers. The easiest path but most resisted by others. Therefore, it’s the toughest path... For the student and the teacher.