Is qada' (making up the fast) or kaffarah (expiation) obligatory for someone who drank water for suhoor after the time for imsak (stopping eating and drinking) due to ignorance of it, while completing the fast for that day, and is there a sin upon him?

The jurisprudential rule "no consideration is given to a conjecture whose error has become clear" means that your conjecture about the night remaining is not to be considered if its error becomes evident. The majority of jurists hold that you must make up this day, as long as you are certain that you drank after the break of dawn. There is no sin upon you because it was not intentional.