Hey Mohammad 

No you are not eligible for the JEE advanced this year because only two consecutive year ( one when you are in 12 appearing / result and another on is in next from the result year).

But you are definitely eligible for JEE mains .

So try to secrls good rank in JEE mains and get admitted in good college.

Good luck 

No, in order to sit in Jee Advance exam, you have to pass your board exams

Yes you are eligible to appear for JEE advanced 2019 if you appear for board in  2019 because the year in which you pass the class XII exam will be considered for JEE advanced there for you don't need to worry you can definitely appear for JEE advance so prepare well for it

All the best