On form controls, not about navigation, about activation. They're both there. Can separate movement to from activation. Don't imply behaviors. To the extents scripts are supported, the issue only partially arises.
Extra links is bad HTML use.
Yes, allowing device-independent access will eliminate the problem. Greg Lowney raised this. This is a level of details in techniques we should push. For data tables that have two or more logical levels of row or column headers use markup to associate data cells and header cells. It occurs in nature, in combo with other flaws.