1.1. Historically, fairly ordinary all round; economics would probably scale the best out of those. Note, scaling is carried out afresh each year.
1.2. Inherently, it would scale better since the Continuers cohort would be stronger than the Beginners cohort. Scaling is dependent on the strength of the cohort - the stronger the abilities of the students, the higher it will scale.
3.1. Possibly. Only you can tell. You might struggle with 10 units or you might find 13 units easy.
3.2. Yes of course.