Absolute Zero Impaling Structures

OK, summarizing what I read in other threads,

1.  He is countermix limited to 12 structures.

2.  The structures can stack.

3.  They are triggered the moment anyone moves or his pushed into the hex, plus once in each character's To Be Continued phase if the character is standing in a hazard.

4.  They do not trigger upon creation.

5.  All ice patches go away if Absolute Zero switches active cards so that the card which allows creation of Impaling Strucutres is no longer in his three-card hand.

(1) Is this all correct; (2) is there any other way Impaling Structures go away?  Can they be attacked with powers?  Do they disappear if stacked with lava?  Can Absolute Zero remove an Ice Stucture at will without swapping the card out?  (We were playing yes on this last point but I now think we did it wrong.) (3) if multiple structures are stacked in one hex, does EACH structure get a separate attack?

Thanks,

--Brett Turner