If Seeker proceeded with that attack, he would have surely lost. And there was even a high chance of getting killed. That attack was just too crazy. At Seeker's level of skill in controlling three different Paths, it was laughable to believe that Seeker could defeat it.
Even if he used these skills to fight back against Lennox, Richie, or Meng, Seeker would have lost because of his lack of control. When Seeker caused the three powers to explode out, he only had to control where they would go.
The previous clash, which resulted in a stalemate, had very little to do with Seeker's ability. It was the power of the three items he had.
And the reason Seeker attacked in this manner was because Seeker was using another Path that he had been practicing and wanted to attain. Cliff proved that it was possible, and seeing that allowed Seeker to believe.
Greg first appeared on chapter 158. He was one of five people who held out the scorecard and the Adult Seeker mentioned him in passing.