This is because gs has a highest possible amount of healing it can do. So if a person is at 10k health with a 400k debuff and something hits them for say 15k, they're health will be brought to -5k, at which point gs will say "I need to trigger" and heal them for 50% of their health (we'll call this number 75k) which will be consumed by the debuff.
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've tossed out maybe 100 GS'es on our Spine attempts and I've never had someone die through GS. Almost always I save it for people under half health that get gripped and have searing plasma, but you can also toss it on people with extremely low health with searing if a blood will kill them and you don't have a spirit link totem going out.
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!