Is it common nature for Minis to have inside tire tread wear (toe out)? I have 15K on this set and they are always rotated every 3.5K. The suspension is fully stock.

 

Yea it is. They've got a few degrees of camber built in. Back in the good old days it was part of what we called the bull dog stance. You're doing the right thing to rotate periodically. I do so at 5K along with oil and filter.