actually, i would argue that not maintaining a fixed sparsity is the whole point of the system. indeed, you are right, the SP cannot be used canonically for this system to work. in the code which i will be posting soon, i completely redo the SP, to make it more in the style of TM, as you described. turns out, this actually simplifies the code quite a bit.
on a disconnected but related note, my code does not model columns in either the inference layer or the pooling layer, whatsoever. i do not think these are functionally relevant in the cortex.