Can a non Div 1 account focused on marquee and legendaries dominate?

Answering the question, yes, it can. See the former Bus74. But that requires GP manipulation to an extent where it halts TW, Arena and TB progress.

However, the moment you did just one unnecessary r7, that strategy falls apart and you are better off going for GLs. The whole key to non-GL GAC play is to keep everything as low as possible, but high gear enough to perform well (r0s on all key toons preferred).