hi, the negative inductance value is because of current direction, the inductance value should be all positive if the current direction are the same. For the equation used to calculate inductance, you can refer this paper: J. Pleite, etc. “Modeling of Magnetic Components based on Finite Element Techniques”.
There are two default boundaries for Maxwell 3D project, natural and neumann. There is no default boundary conditions assigned to simulation region for Maxwell 2D, user have to specify it separately.