Please reread it.
In no place does Aaron (Peace be upon him) admit to idol making and/or worshipping. In fact the people go near to slaying him! (Surah 7:150). Aaron (pbuh) is only guilty of not wanting to split up the Children of Israel.
As for the sequence of Moses returning and the Children of Israel repenting, there is no conflict. In Surah 7, verses 149 and 150 both start with the Arabic "wa lammaa" which means "and when(at the time of)" or "as for when". This is not denoting sequence in time according to the Arabic.