Can anybody explain me what is meant with the LL's accountability: "Structuring the governance of the circle to express its purpose and enact its accountability. "
to me it means: "make useful Governance proposals, so that the Circle's Governance makes more sense."
You are right that every role processes tensions. The Lead Link has the "extra" job to make proposals that tend to the entire circle's purpose, e.g. to propose new roles to express the purpose of the circle better, or to make changes to several roles so that the circle functions better overall. I take it as a kind of "general" permission and encouragement to propose improvements, and to sense tensions in all areas of the circle's operations.