For a refua shleima for Moshe David ben Devorah.
Yichud - some poskim prohibit adoption because it will lead to yichud problems with the parent of the opposite gender. I understand Rav Moshe Feinstein in Igros Moshe E"H IV 64:2 as holding there isn't a problem to adopt a girl because there is a super din of "ishto imo" so there is no concern of yichud. His heter apparently would not apply to a couple adopting a boy, because of the yichud problem between the adoptive mother and the boy. I previously mentioned that my rebbi zt"l paskened there is no problem of yichud because Meiri Kiddushin (81b s.v. Kevar) says: "...However, whomever does this without the intent of yichud and for unclean thoughts, rather, the way of the love of the ones who raise a child (derech hamolidim) to publicise and make known that their children are very dear to them, it is permitted. And on this it is said: hakol lesheim shamayim." He expanded on this and said from Tosfos, Sanhedrin 37a s.v., Hatorah, that sometimes a the thorn around roses - sugah bashoshanim - is a stronger barrier than an iron fence. The sugah bashoshanim of the child-parent relationship in case of adoption will form a psychological barrier against issur, and there will not be an issur yichud.
Writing name on Kesuva - How do we write the name of an adopted person in a shtar (contract)? Rama in Shulchan Aruch C"M 42:15 allows one to wrote "ploni ben ploni" even when it is only an adoptive relationship. For an aliya, most use the adoptive father's name, and some say ploni hamegadlo (who raised him). In a kesuva, the old custom was to write the biological parents (when they were also Jewish). I have heard that R' Moshe Feinstein said they can write the adoptive father's name without hamegadlo. (See also E"H I:99; E"H IV:26). Rav Yitzchak Yaakov Weiss in Minchas Yitzchak is very much opposed to using the adoptive father's name in a kesuva and even for calling the boy up for an aliya, because he is worried that people will make incorrect assumptions by hearing him called up as ben ploni. See Minchas Yitzchak I:136, IV:49, V:44, and VI:151. (He also prohibits yichud in an adoption).
(You could make several distinctions: an adopted non-Jewish child who underwent geirus katan: should he be called ben Avraham Avinu like a Ger ,which s/he technically is, or ben ploni hamegadlo? Because what about a non-Jewish man married to a Jewish woman and later the father converts - especially in a case where the community does not know the father wasn't Jewish, could be call him up ben ploni hamegadlo? What about where the father was Jewish, the mother wasn't, and then the mother & child convert? Can we use hamegadlo or must we use ben Avraham Avinu? (I think Nachalas Tzvi would allow us to use hamegadlo in this last case.) Also, an Igros Moshe EH I:99 has a case where nobody remembered the boy's father's name. Probably that case was where his father died when she was very young and her mother remarried, and this man raised this girl. Rav Moshe paskened to write bas mi shenishkach shmo venikra ben ploni shegidlo. I mention these possibilities because there are several distinctions we can make, plus there may be social considerations where to use the word hamegadlo or shegidlo will cause scrutiny.)
A frum family had to give up a child for adoption, and the child was adopted by another frum family who allowed the child to have contact with his biological parents. When the child got married, the biological parents insisted on walking the child to the chuppa, while the adoptive parents who raised the child wanted to. They had a Din Torah by my rebbi Rav Ahron Soloveichik zt"l. He paskened, based on the Shemos Rabba 4:2, which states: When Hashem told Moshe 'go now, I send you to Paroh,' Moshe responded I cannot, because Yisro accepted me and opened his home to me and I am like his son, and one who opens his home to his fellow, his soil is obligated to him...and more than that, whoever opens his home to his fellow, he is required in honoring him more than his father and mother... So Moshe told Hashem Yisro accepted me and honored me, I cannot go unless he gives his permission. From this Midrash, my Rebbi paskened that the adoptive parents who raised the child should walk the child to the chuppa. Rav Aharon Kotler agreed with this psak.