I would answer your query one by one.
1. Form for improvement exam would be available from September or October. It may be available in selected Syndicate Bank branches of where you have to fill the form and deposit the requisite fees.
2.Yes you can give the exam in five subjects. But I think PCME is compulsory for JEE. 5th subject is your choice.
3. If you fill the form early, then your exam centre would be inside the 5km radius from your home. So try to fill the form early as possible. Also it would be much cheaper because as time passes, CBSE puts late fee along with.
4. Now for the admit card it would be available on Cbse website from February 10 onwards and if you have done through the process correctly, then you can download your admit card. If not then you have to visit CBSE Preet Vihar Office to do the corrections.
As for your marksheet, you have to collect it from your examination centre of your improvement exam.