Admissions won't be based on a first-come-first-serve basis. Aspirants have to meet the cut-off criteria for a particular programme/course and then will be considered for admission as per the schedule.
No, there is no offline form for registration.
Once you are admitted in a particular subject, you can change your subject only before the last date of admission provided you have applied for that course in your online registration form, meet the requisite cut-off and provided seats are available. You cannot change your subject during the course of study or after completing first the year of your study.
No, applicants with gap year(s) will not be at any disadvantage for the purpose of admission.
You can apply for ECA and Sports quota by selecting appropriate options while filling the registration form on the DU Admissions portal.