Rashi explains that Mipnei HaYireh means that one who is actually afraid of his life can answer.

This mishna seems to contradict the mishna in the Fifth Perek, where it says that even if there is a snake sitting there ready to strike, you cannot move.

Is the difference that one is shma and one is shmone esre?

Oh, and mazel tov to those who finished the first perek thus far

Im not sure but I think that in Perek Chamishi it is talking about a snake that is not life threatening, and there is no danger of life involved.
The Gemara over there actually comments that "Lo Shonu Elo Nacash Aval Akrav Mutar" That if there was a scorpion around his heel it would be permitted to answer him.
I mean obviously we are not talking about a case of Pikuach Nefesh - where there is danger to life - after all Pikuach Nefesh Doiche Kol Hatorah!

With regards to the actual question - there are obvious differences between different parts of Davvening, but I am quite sure that if there is a possible danger to life than one could always answer - based on the Rule 'Pikuach Nefesh Doiche Kol Hatorah Kulo"

That is exactly the difference.