As for the binding expression errors, I think they come from the leggings model you have going, but it is different from the one I uploaded.
In the one I uploaded, I just named the groups the same as the ones on the Vanilla player geometry (root, body, rightleg, leftleg), which is an alternative way of making attachable groups follow the wearer's geometry. They don't need binding expressions then.
You have renamed 'body' to 'body2', so that one won't follow the Vanilla 'body' group.
You must have binding expressions on each group since you get errors about that, and I believe it is because you used the names of some armor model, instead of the player model's groups.
There isn't any 'rightlegging' or 'belt' group on the player itself, you have ot reference 'rightleg', 'body' etc