Suppose I find myself in a situation where I am not sure whether my life is sufficiently in danger to warrant halachot of pikuach nefesh to come into play. What should I do in this case? Am I supposed to (e.g.) violate Shabbat in this case?
To be clear, I am not talking about a situation where I do not know how much danger I am in (I am having sudden chest pains and I'm not sure whether it's a heart attack or something more benign. My understanding is that this is the category of safek pikuach nefesh and it is treated the same as pikuach nefesh situations). I am talking about the situation where I am aware of the level of danger and I have a safek about whether the risk is sufficient to constitute pikuach nefesh (e.g. I am caught driving in a severe blizzard and Shabbat is about to begin).
“One must desecrate Shabbos even if there is only a slight possibility that the situation is dangerous. One does not need a professional opinion or an expert physician. Whenever one is uncertain whether the situation is dangerous, he is required to desecrate Shabbos (Shu’t Tashbeitz 1:54).”
By all means !!!
Don't even consider a "Pikuch nefesh" situation as "Hilul Shabbat" - but act without delay in ANY way you can to save a life. Even if it is only a slight suspicion. And NEFESH is not only physical, it also means Mental or a worrisome situation.