Eric,
I'll go out on my own limb, and say that wear is NOT on the inside as you posit, but actually on the outside. Kurt can correct me if I'm wrong, but in the pic of the tire straight at the thread, that is typical non-rotated tire wear on an open type tread. In the absence of alignment, balance, brake or suspension issues, it comes from cornering forces, or "scrub".
Open treads are indeed more susceptible to all types of uneven wear, including those from alignment, balance, brake or suspension issues.
The trouble with wear patterns on tires is that once they start, they cannot be corrected even if the source (alignment, balance, brake or suspension) is fixed. A vibrating tire from a wear pattern is indistinquishable from an out of balance tire from the driver's seat. And a tire thus damaged will always be problematic to balance and the balance will not last. The way to prevent a wear pattern from manifesting itself is through aggressive tire rotation, and yes front to back on directional tires will prevent a wear pattern from developing.
It's expensive, but if you want to have a smooth running rig, starting over with new tires and being aggressive with rotations may be the only way.
Dave - if your xterrains were run out of balance long enough to develop a pattern in the rubber, they're never going to run smoothly again. Period. If your mechanic told you the uneven wear was due to the tire's design then he was smokin crack. Barring a defect in BOTH front tires (a statistical anomaly), they were mistreated in some fashion as noted above.
Rotate. Rotate. Rotate. At the slightest HINT of an imbalance, get them balanced immediately.
DougM