Hi, I am sorry that I am not sure about the problem. For your first question, exactly zero vN entropy means the direct product state. The only possibility I could imagine is that you don't establish interaction between two blocks. Which means your Hamiltonian in odd sweep is empty. You may check whether you build MPO correctly.
For the second question, I don't understand why you have "Nx". Are you really using idmrg? If you are using idmrg, as Miles said you cannot use AutoMPO, instead you have to DIY an MPO. And when constructing MPO, there is no concept of "Nx", you only have "Nuc". You may choose Nuc to be your Ny, thus system grows layer by layer, and the number of sweeps tells you the final Nx of the system. You may check the Heisenberg.h, it is a helpful example.