Sir,
Kindly note that in composition scheme you would not be allowed any input credit nor your buyer will be able to take input credit. So, even if you are opting for Composition from 1st Jan 2018, all your input credit on stock will get reversed.
Regarding filling of return, till September ;late fees have been waived off and after that there were many changes for late filling fees slabs.
Regarding your query for reconciliation, it can be examined and then replied.