Commercial dance is more of the advertising or business side of dance. This major prepares you more to be employed as a dancer in music videos, tours, productions, etc. It’ll explore more “popular” types of dance like jazz and musical theater.
Regular dance will be more of a preparation for being a professional dancer in a company or as a teacher, and will focus more on one type of dance than being interdisciplinary.
Depends on the type of degree. A BA in dance will likely be less time-consuming than a BFA. Almost all BFA majors don’t do double majors. A BA could let you do a double major, but it’ll be a lot of work if you’re not doing a double with another arts major.
Depends on the school. Schools typically will not give scholarships or recruit for dance since it’s not a competitive sport for most schools. For auditioning, most schools will publish what they are looking for on their website. You can go ahead and contact the coach, but it’s not like they’ll recruit you. You’ll have to audition like everyone else. Most students on competitive dance teams don’t major in dance. The top teams in the country (Louisville and NC State, for example) have mostly non-arts majors.