Hello,

Yes you can appear for improvement exam and jee main has given this option to candidates. There is a block in jee main application form there you have to fill all that you have appeared for improvement exam.

You have a good chance to improve your aggregate so prepare well and give your best you score more than 75.

Regarding the marksheet you should talk to your intermediate institute that within what duration you receive it after appearing improvement exam.

The result which have high value will be considered at the end so don't worry if you score low. Just give your best and prepare your basics.

JEE mains counselling will consider the certificates so don't worry about it. Just try to get your certificates before your counseling process else you will get rejected then and there as you won't have the accurate documents for verification.

You can check what are the mandatory documents required and make them prior the counseling process so that you don't miss out any needy credentials.

Hope it helps!