Chesterthemany

I believe, that since they are Insignificant, they are ignored for the part of the Strategy that gives them the action. However, the effects of the action are from the model - so as long as a SIGNIFICANT model performs the action, I think they would get the heal (or damage).

Correct. Laugh Off prevents other models moving that model, but Obey forces that model to move itself. The only thing that stops Obey effects are things like Unyielding.

So the trick here is, it's an ENEMY'S vent steam that the GRAVE GOO is inside - ergo, he's counting it as terrain, and then spreading HIS aura to the terrain - making the enemies suddenly also count it as terrain.

Yes, the auras count as terrain pieces. The only thing that might prevent her from TARGETING a terrain aura is if SHE counts it as terrain - for instance, some (like Field of Steel) say ENEMIES treat the aura as hazardous (or other things) - since she's not treating it as hazardous, that aura has no terrain traits to her, and thus, is not terrain. Other than that, if it has terrain traits, you target it all you want. Now, if an aura says friendly models IGNORE THE TERRAIN TRAITS, she can still target it. She's ignoring the traits, but she still sees it HAS them.