I submitted a series of questions to Perry mostly related to the topic of this thread; I will of course share his reply when it comes (here, and in the "perry sez" section).
So, resurrecting the thread
I got Perry's answers and they seem to make things pretty clear.
A copy of the questions and answers is here
(Perry Sez thread)
What this seems to boil down to:
* in a moving stack, some units can be using Assault Movement while others are not (possibly because they don't have the same MF, or possibly because some intend to expend more than others)
* in a moving, concealed stack, if some units perform a Concealment Loss activity, only these will lose Concealment (think "smoke grenades" or "DC placement")
* some units can leave the stack voluntarily
* if a unit in a stack rolls a 6 on a smoke placement dr, others can keep moving.
* if a unit leaves the stack, it doesn't mean its movement is over, it can resume moving after the rest of the stack ends its movement. Of course if the stack contains a leader that remains with the stack, later movement will be without the leader bonus.
This seems to make a lot of nice tricks possible, some of them involving moving next to the opponent under concealment, laying smoke (either in own hex or on the opponent), then a leader placing a DC under cover of the smoke - but without revealing the DC before the smoke is actually placed.