OK, meanwhile I looked up these Amiga-only levels from Mindless's package and put them together in a levelpack file. Anyone intrested to have a look, feel free to download it:
http://downloads.pigsgrame.de/AMIGALVL.DAT (Should work with the customized Lemmings game as well as with Lemmix).
"Going their separate Ways" was not included in Mindless's upload, but I downloaded this level several years ago from a source I don't remember. Because it was not extracted directly from an Amiga version, I cannot verify that is the original one, through it certainly reproduces the original idea.
I have to say that these are very intresting, challenging levels. The "special" levels which were added in favour of them have, of course, better eye candy, but I think the dropped out ones clearly grant the better gaming experience. If I had been in the developers' place, I would probably have dropped some of the less interesting doublettes and kept those Amiga levels. As for "Going their separate Ways", I have no idea why this was exchanged by "All the 6's". The latter might be a funny idea, but is not challenging at all, whereas the original one offers a unique puzzle.