1. It will be valid till March 2019 the certificate is valid in financial year 18-19.
2. It would be same for the financial year.
3. States give there own notification for calculation income in income test in obc NCL different states have different limits.
4. Sir firstly the above certificate is not valid for five years secondly the rules are different in case of State and Central OBC though authority is same.
5. The certificate won't say you are NCL from last 3 years it is effective from.date of certificate and most of time we get it done on same day.