Do the thousands of children adopted as babies grow up with attachment disorders? No, their new parents are given professional advice on how to overcome it and work hard at establishing a relationship. My nephews were much older when they were adopted by dsis and have developed strong and secure attachment. Any issues they do have are a result of their neglect in their early years, but even those are diminishing as the months pass. Honestly, it is not going to cause irreparable damage to the op's dd, providing she returns and works hard at re-establishing a secure attachment. But op does need to understand that she'll have to work hard when she returns and may find it emotionally very difficult.
As the adoptive mother of 4 children (and another 4 children who were placed on a permanent fostering basis) who were placed with us as babies/toddlers I would agree with some of what you say, especially about having to work hard, work VERY VERY hard on the OP's return. You also made me laugh at the professional advice as certainly did not get any! And if we had it would have been textbook stuff given by professionals with no real day to day experience of the situation.
Every situation is different, but my limited experience has taught me that many/most babies moved from their primary carer at the age of the OP's little one DO have great difficulties. Difficulties that of course can be overcome, but difficulties nonetheless.
I do accept that your lovely nephews problems are diminishing, that is great, and most children do, mine too, but honestly these problems/difficulties are very likely to re-emerge during those difficult teenage years, or perhaps when becoming a parent themselves. Of the 8 children we have seen through to adulthood, with whom we have wonderfully close and loving relationships as adults, and are my best friends as well as sons and daughters, I have seen problems resulting from the past they cannot remember re-emerge and create many difficulties for them emotionally. Not one of them is untouched by their early experiences.
I have also been a foster carer for 20 + years and again I truly believe this particular age has the most problems when separating from their primary carer. I realise this sounds dramatic, and I certainly do not believe the OP's little one will have any major, long term problems, but I do believe there will be SOME problems. Most people may well disagree with me, and I accept that, I am just talking honestly from my own experiences.
And so because of my own experiences I would always advise against being apart from your child at this particularly vulnerable age. I also remember fostering a young baby between the ages of 9 to 14 months. She had had such a wonderful bond with her primary carer, from birth, and had only known love and security. When placed with us it was so so difficult as she just could not bond, despite trying almost everything. We eventually did, of course, but when she was able to return to her first primary there was an awful reaction and an even longer time taken to re-bond. Those problems came back with advangence when she was a teenager, again the problems were overcome, but a difficult time for all the family nonetheless.
But for me as a mother who has missed out on my children's many firsts (before we adopted them) is of great sadness to me, and I would give a million pounds to witness their first words, first steps, first smile.
Sorry if this is a bit off topic.