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1) Ken zayn the minhag is/was not to wash mayim achronim at the seder. Rationales for minhagim are easy to make up: since mayim achronim is primarily for melech sedomis causing harm, on leil shimurim there is no concern
2) Youll note I never said it was the strongest rayah
3) I’m not sure how long the “minhag” of wetting fingers for mayim achronim is, but while I’m sure there are people who actually wash their hands IVe never actually seen it.
4) Not being included in the simanim is not the same as not being in the haggadah. Dont you find it strange that (almost?) every hggadah has kiddush printed incuding Vayechulu yet rarely (if ever) does it have Sholom aleicehm?