We don't and will never know the ins and outs, because social care (rightly) are not allowed to comment. It would breach the confidentiality of all affected.
So social care might have got things massively wrong, or there might he a huge backstory showing they were right to put the plans in place they did, but we won't ever know.
From the little we do know, it seems like the partners solicitor did the obvious harm, by mention adoption in an email. The family shouldn't be told their child might be placed for adoption like that, but that was the solicitor's mistake, not social care.