In the time you have available, under the conditions you have the candidates, I don't think I would be able to determine competency or even personality traits.

In a 4 week segment that I taught I would start to form opinions of students after ~6 hours of interaction with them. Sometimes arrogance was just nerves, expectations were super malleable and not necessarily deeply embedded.

Competence is almost completely separated from course completion / rate of completion. Effort applied isn't a great indicator either (although slightly better than the other two).

I would recommend if you want to find great candidates, teach a bootcamp. There's literally the top 10% going through that can just be picked off. I hired one of the 3 brightest I taught and the other two I primed for when I have hiring budget.