ANSWER:

The inheritance is halal for you all as the haram money is only haram for the person earning it and not for anyone else. 

If he had not fasted deliberately without any valid reason then you must not fast these on his behalf.

My father passed away, please make dua for him. Most of his life my father did not pray, nor fasted nor paid zakah and accumulated interest on his wealth. Now what should we do as his inheritors first? Will the wealth inherited be halal for us if Zakah and interest is not taken out? How shall we compensate for his fasts? The calcuation of 40 years is hard.