online

Offline is better at the first instance, because we have been appearing in Offline exam from childhood and this mode is comfortable.
It is noteworthy that JEE Advanced will be in Online mode only. So, if you choose online mode in mains, it will help you in long run.
Otherwise, you would need to appear in Advanced in online and you might not be well prepared for this experience. Nobody wants to mess up such opportunity.
Another point, if you choose JEE Mains offline you can change it later in online mode. However the reverse is not possible.
JEE Mains Preparation tips: https://scoop.eduncle.com/jee-mains-preparation-tips

Apologies for the late reply! Unfortunately, no one can fill the application form of MDI Gurgaon as the last date has already passed by(Nov 24). Still, if you had a very genuine reason for unable to fill the form on time, you may contact the college officials and address your query.

Hi,
You can take only one exam that too which mode you have specified while filling the JEE form. However, changes in the form can be made. For that, visit the website of JEE Mains.
Thanks.
Keep asking.

Please download edx or coursera apps from Google play store. They have so many free courses for you. Those are recognised courses. With little payment, you can get certificates of well known universities like Harvard and Edinburgh, etc.
All the best.