1) No. of seat can well be checked by source as NGP & Destination as MAS
2) Quota exists for New Delhi - Nagpur / Nagpur - Chennai bookings and various other combinations
3) U'll have to contact TTE on-borad & in case of vacant berth, he may upgrade ur ticket
4) The difference in charge suffice the difference in fare, but may vary...
more... slightly, depending on other issues. Better get receipt for the charge u made, TTE will never be able to charge u extra in lieu of receipt!!