Was king Bumi already a member of the Order of the White Lotus when Aang first went to Omashu? Why didn't he thought of teaching Aang earthbending then?

Then I do not understand why Rouku wanted Jeong Jeong to teach Aang firebending?

Rouku was not an avatar without mistakes. If you remember in his time he made some bad decisions I doubt he changed that much by the end of his lifetime. Also, like i said, Jeong Jeong failed to teach Aang because he was not yet ready.

In most cases any Avatar would have to learn the elements in order, however, since they were pressed for time, Roku may have been grasping at any opportunity for Aang to learn from whoever was avaialbe at the time-even if it was out of order, which made things even more difficult for Aang

Because Toph was meant to be his teacher not Bumi just like Zuko was meant to turn good and teach Aang firebending. And Roku made Jeong Jeong teach Aang cause he was momentarily the only person in the fire nation who wasnt evil