Thought I'll clear up some things:
Heavenly Cataclysms are thought to occur since the beginning of the world since they have been happening periodically and since its been so long, no one really knows when it first started. However Cang Wuzi is doubting if Heavenly Cataclysms actually did occur in ancient times since there were so many powerful late-stage cultivators across all the races that seemed to co-exist. There shouldn't have been enough spiritual qi if heavenly cataclysms did occur.
His question regarding higher-stage cultivators is just why are there no records of them at all.
Yes, he killed the other cultivator and himself both postpone the heavenly cataclysm and demonstrate his own theories. Also note that the 'Proxy of heaven' group currently does wish to kill a lot of mortals to overcome the heavenly cataclysm. Cang Wuzi probably chose Chong Ming because he disagreed with this ideology (and possibly also to help his disciples). He is the original leader, however he's being in seclusion for a long time and now the group has changed their purpose to be a bit more dangerous. Some of the original 'Proxy of heaven' members are likely pretty chill, e.g. the Kunlun sect leader.