They will live if current health + gs heal > amount remaining on debuff.
GS is still invaluable for the +60% healing taken and the ability to set off a large heal before a person dies. If the debuff doesn't have much left, chances are GS will save them.
I'm not clear on what you mean here. Movement speed increases do not stack. It has been the subject of countless tests since Vanilla and can be verified in the Character pane these days. BaS is also a static 160%. All other increases are ignored when it's active.Make sure to cycle through stances for the ice phase. Ditch inner focus and throw on inner will for the extra 14% movespeed (6% extra movespeed if you have run speed on your boots, which you should) but be sure to pop inner focus again before the end of the phase!