Main problem: Should I say some answer was fine? I think sadly, not. I'm not sure to check on my answered question as ok because it seems that, unintentionally, I made a trick question: cause the answers are always going to be flawed.
That is, on the one hand, it's possible to get a scaled result as required, and it's also possible to not change the original pdf... However, on the other hand, to keep design proportions it'd be needed to eliminate too much white spaces in the margins (literally cutting with scissor/cutter the physical paper off). This was not intended as end result, but there was no awareness on my side that this w/could happen in the beginning.
Hence, if one wants an a6 paper printed in the end and asks how to add offset for a6 result (keeping proportions! Heck, we're in LaTeX here!) one of two bad things are bound to happen. Or either things get shrinked and out of proportion (yuck), or else the end result needs severe trimming (yikes) which was not contemplated before nor seems acceptable in the community either. How to give feedback on a paradox?
On another note, the post got some nice feedback that was and still is very useful to me. It's a topic that would do well if it can get some good canonical answers. This is why I didn't incline much to deleting the post.