1) “Kiymu v’kiblu… v’al kol hanilvim aleihem” (9:27) Rashi explains “kol ha’nilvim aleihem” is a special ribuy to include geirim.
Geirim are obligated in all mitzvos. You don’t need, for example, a special din to tell you a ger is chayav to observe Chanukah. Why is a special ribuy needed to include geirim in the chiyuv of Purim?
The Brisker Rav (quoted in R’ Turtzin’s Kuntres Chanukah u’Megillah siman 9) explains that the mitzvah of Purim is different because it was only by virtue of a kabbalah – “kiymu v’kiblu” -- that Klal Yisrael accepted the mitzvah. Since even we needed a special kabbalah to become obligated in the mitzvah, geirim also need their own kabbalah.
R’ Chaim Kanievsky in Ta’ama d’Kra (p 220) compares the chiyuv to celebrate Purim to the din of birchas hoda’ah on a miracle. Not only can the individual who experienced the miracle recite birchas hoda’ah, but one’s descendants can recite the bracha as well (O.C. 218). Since future geirim are not descendants of those who experienced the miracle, a special din is needed to be mechayeiv them.
2) The Aruch haShulchan writes in 694:2 with respect to giving matanos l’evyonim:
ויראה לי דאיש ואשתו – שניהם יוצאים בשני מתנות, דכגוף אחד הם.
However you understand it, he applies an ishto k'gufu type sevara to say that a husband and wife can fulfill matanos l'evyonim with one joint gift.
Yet he writes in 695:18
וכל הנשים חייבות בשילוח מנות ובמתנות לאביונים, ואפילו יש לה בעל – אינה נפטרת בשל בעל, דזהו מצוה דרמי עליה
Here he says women must independently fulfill their obligation.
I don’t understand how to resolve the contradiction.