Acharei-Kedoshim 1

A couple of notes on my last post:

1) The Arizal actually popularized Hilula d'Rashbi (the yahrtzeit of R' Shimon bar Yochai), which falls out on Lag BaOmer, but is not an inherent part of the chag (the latter being defined as the day that the students of R' Akiva stopped dying, or some variant thereof). Although, based on my limited experience with kabbalistic matters, there probably is some connection that is brought down between the two.

2) I asked a rav about the usage of an ordinal number rather than a cardinal number for sefirah, and he said that it was not an issue.

New matters:

1) Why does a yoledes have to bring a korban chatas? Rashb"i on Niddah 31b says it's as a kapara for the oath that she made during the travails of her labor never again to have marital relations, to which Rav Yoseif brings up several challenges that are not dealt with by the gemara there (i.e., she was m'zidah, her sh'vuah can be hutar through going to a rav and thus does not need a korban, the korban prescribed for her is not the usual korban for a sh'vuah). Tzarich iyyun.

2) Why is the shiur for hotza'ah of milk on Shabbos only a sip (g'mi'ah) and not a revi'is (like by other liquids) or some derivative thereof (like undiluted wine, for which the shiur is enough to be diluted into a revi'is). The gemara emphasizes that the purpose of the milk is for drinking (or, interestingly (?), another portion of the sugya refers to it as eating), not for any other purpose, such as medicinal.

3) If we allow that mayim acharonim is still in effect nowadays, according to the psak of the Shulchan Aruch, how does barely sprinkling the fingers accomplish this? There are two reasons given for mayim acharonim to my understanding: Melach s'domis (leaving aside issues of metzius), which would ostensibly require a thorough washing b/c of sakanah, and a drash from this week's parsha (for the requisite tie-in) which learns out mayim rishonim and mayim acharonim from the dual lashon of v'hiskadishtem vih'yisem kedoshim (20:7). The latter reason would appear to juxtapose the two, and thereby equate the two into chiyuvim that require a revi'is (maybe this is the basis of the Gra, who holds that even mayim acharonim require a bracha), while for the former reason, one would have to say that only the fingertips would have come in contact with the melach s'domis, which I originally thought didn't sound so compelling, but seems to make sense.

Update, 7/3: This past Shabbos, I was asked that, if the reason for mayim acharonim is merely because of sakana, what's the reason for removing or covering the used water during bentching (presumably because of ru'ach ra'ah). This would seem to indicate that there's some aspect of vih'yisem kedoshim in the chiyuv, also, which then reintroduces the question of why we (meaning the various shuls in which I've eaten seudah shlishis) only wash the finger tips. Unless there's something tamei in general with used washwater/bathwater (and that being the case, a swimming pool that is not a valid mikveh)?