The source looks like it really cannot be saved by any deblocker because there are no details to restore
I've sometimes used quite unorthodox measures with such sources by replacing the extreme blocking with noise.
So maybe something like FFT3DFilter(sigma=5) + GrainFactory3(20,30,40) would help you? I would first use FFT3DFilter, then downsize to 720x576, do the framerate conversion, add noise and finally do whatever you need to do to get the final format you want (depending on whether you are going for case i or ii you mentioned).