Dunno off hand which they are - I think there's a message or 2 on the list covering the topic a few years ago & I'll look it up - but whichever corners they are it is the ones that count as the front corners at the end that matter.
A quick search found this -
http://games.groups.yahoo.com/group/DBMMlist/message/40435 Doesn't answer the question about unequal ranks, but does about forming columns
I haven't found anything specific, but I'd play it that they are still the front corners of the front rank.
this will disadvantage such a formation compared to having a formation where the front rank has at least as many elements as each subsequent ank - elements are still all limited to moving the same speed as the slowest element. So that rear rank element out on one flank can only more that far - if hte formation wheels then I'm prety sure geometry will then require that the front corners of the front rank must both move less than that distance - which will invoke the PIP penalty for "moving short"