Thanks Gang. A suggested future addition, to make this more straightforward, is to have a 3dLM version of 3dLMEr (or even better, allow robust linear modeling as well). But that may be down the line. In the meantime, a quick question about 2).
For 2) Remove the population mean of 'brain' at the voxel level and call it BRAIN;
Is that a 3dcalc command in which mean is the mean of all brain voxel b-values?