more on shluchei mitzvah ainon nizokin -- chakira of the Sdei Chemed

I) I saw the following chakira quoted from the Sdei Chemed: when we speak about shluchei mitzvah ainan nizokin does it mean that Hashem removes the threat of danger, or does it mean that the threat is there, the danger is there, but Hashem does not allow the person doing the mitzvah to come to harm?

In YD 339:5 the SA quotes a minhag to spill out any drawn water in the neighborhood in which a death occurs.  Some explain that this was simply a way of communicating to all what had occurred without having to actually announce the bad news.  Others, however, explain that there is a supernatural reason for the practice.  When the malach ha'maves marches into town, some kind of poison can drop from his sword into the water and render it dangerous for anyone to drink.

In accord with the rule of shluchei mitzvah ainan nizokin Rama in Hil Pesach (455) writes that mayim she'lanu that is used for matzah baking does not have to be spilled out.  The same (see Pischei Teshuvah in YD) applies to water drawn for shabbos food.

Can one eat matzah baked with that water after pesach is over?  Can one eat the leftover cholent after shabbos?  This should be taluy in the Sdei Chemed's chakira.  If the principle of shluchei miztvah ainin nizokin means there is no threat present, i.e. no drop from the malach ha'maves' sword ever falls into mitzvah water, then the answer would seem to be yes, one can eat the leftovers.  But if the principle of shluchei miztvah ainin nizokin means the drop of poison is there but the mitzvah protects one from harm, then after pesach (or even the seder) is over, after the seudas shabbos is over, once the mitzvah is complete, the leftovers cannot be eaten.  

II) The context of shluchei mitzvah ainin nizokin in Pesachim is a discussion of the braysa תנא אין מחייבין אותו להכניס ידו לחורין ולסדקין לבדוק מפני הסכנה.  The gemara gives two explanations why a person would not have to search these literal holes in the wall:

1) Lest one be bitten by a scorpion.  Asks the gemara: והא אמר ר' אלעזר שלוחי מצוה אינן ניזוקין? To which the gemara answers that maybe a person will accidentally drop something of theirs in the hole and search for it after he is done with the bedika, at which point they would be exposed to danger.

2) The house adjoining the wall belongs to an aku"m who is might not understand the bedika and think it is some kind of witchcraft, which would be dangerous.  Again, asks the gemara: האמר רבי אלעזר שלוחי מצוה אינן ניזוקין  To which the gemara answers that this is a case of clear and present danger, שכיח היזיקא, and all bets are off.

Why does the gemara not gives this answer of שכיח היזיקא when it talks about why there is no protection from the scorpions?  

It could be that these are not just two possible scenarios to explain the braysa, but rather there is a difference between the views.  The Ohr haChaim writes that Reuvain thought it preferable to throw Yosef into a pit of scorpions rather than have his brothers deal with him because scorpions have no bechira and could harm Yosef only if Hashem decreed such; the brothers had bechira, and Hashem allows bechira to play out even if it goes against the ideal Divine plan.  It could be that shchiach he'zeika only comes into play when one is dealing with a human agent, a baal bechira.  When it comes to animals, since they are like robots that cannot deviate from Hashem's plan, even if shchiach he'zeika one is still protected.