I think there was an interview where broncos players basically said mariner and piakura were both space cadets.
Reading of the attack is so important in Edge defence, and mariner just did not demonstrate those skills. The nrl is not the greatest place to learn the basics/essentials. That is why the broncos were not successful with the "potential" young players. Yes they could be good, but they have some big holes in their game that first graders identify and exploit.
Previously many centres were introduced to top grade via the wing, and would learn working next to a more experienced centre. This is why I think he is best playing the wing for now (at least defensively).
With regards to mam, In Qcup, mam's defensive reads were terrible, but when in first grade, was positioned next to capewell and herbie who were both pretty good defenders/readers of the attack, and this wall papered over mam's deficiencies, but when they left and he was placed next to mariner and piakura who both had their own defensive issues, the flood gates opened. That was one of my major issues with KW coaching, he did nothing to address the defence on that side.