Coming to the Southern Federation was just a spur-of-the-moment decision, entrusted by someone else. However, Li Feng's character is such that he never leaves empty-handed; how could he return from a trip to the Southern Federation with nothing?
The Oppenheimer family simply happened to be in the wrong place at the wrong time. It was just their bad luck.
As for the relationship with the Gods Temple.
Of course, they are still enemies, the kind who wish for each other's demise upon meeting. But when there's no chance to eliminate the other, and there's money to be made, no one would pass up a profit.
Thus, everything that happened came about as a matter of course.
Li Feng played a game of half-truths and deceptions, and the people of the Southern Federation, who knew nothing of military strategy, were naturally duped.
Even if they realized Li Feng was behind everything, the final outcome would be the same: they would admit defeat.