Culturally I think names are really powerful. There is a whole essay in this! Names as destiny, names as connection, names as indicators of class, ethnicity, ideology and family. Obviously names are powerful, and it's right to consider this deeply. Even on a personal level, the voice of names can be so profound. My family still tell how I was known as something different for the first 3 weeks of my life, what inspired the name choices, why my brother and sister have long names that can be shortened but I don't, which grandparents put pressure on then to use or discard a name. The process of choice is a reflection of my family history, a snapshot of a moment when certain relationships came to the fore within my family. It goes down generations too- my siblings children have been given family names that reference each other, but not me. It stings, and it isn't even that important. It isn't even my own name! Maybe the new style of name represents BPs determination to do something differently? A break with the other type of name to demonstrate a commitment (even if failed) to be a different type of parent for this child? That is symbolically a powerful thing. I would be careful about what family lore and narratives are being created if you decide to change a name you don't really have to. What does it reveal to your child about your relationships to them, the BPs, the world? Which part of their lives are you deeming too difficult to explain, too prickly to handle? There is no right answer, but you will need to have an answer because your child may have those questions.
I would have so many questions that inform my thinking. Do you know whether BPs have any reasons of why that name was chosen? Have social workers done the risk assessment of being found, is there any proposal of direct contact, does the name have significant meaning for BPs? I think that might shape my choices in your situation. If we are talking from Norse/Viking mythology, it could be a nod to northern heritage, although I know those names are popular now. It might represent a specific film they saw with a loved one, or even just think sounds beautiful to them. Alternatively it might represent a connection to a certain class or heritage, and by the sounds of it you recognise it is maybe not such a middle class name. But a working class heritage is not to be ignored, and could be quite significant to the child later on.
To me, my discomfort with a name I wouldn't have chosen myself is something I would bear rather than the burden of my child knowing that I had deliberately changed a part of them and their history for them to seem more palletable to other people/myself. Personally, I would try not to prioritise what other people think, it is a connection to BPs that isn't a strangers right to give or take away. They aren't the ones that will have to reckon with the disconnect in the future. As other people have noted, lots of older adoptees talk about the significance of name changes, and I haven't found many that have felt like it was an affirming act in the long run. It may be easier in the short run, but it also positions you and BPs as diametrically opposed- obviously they generally didn't make great choices, but overall, I would say the naming of a child is pretty sacrosanct. Rarely is it done lightly, and our social worker hit the nail on the head when she noted to us that often parents have to grieve the potential names of children they might have given birth to as part of their acceptance of the realities of adoption. Many people hold in their heads that their child will be a certain type of person because of the name they give them. We expect different things from a Chardonnay than an Olivia. By accepting a child with a name you wouldn't have chosen, fully, openly, being proud of it for them, in many ways you are signifying you are open and proud about their history and story, not backing down from the difficult bits or refusing to acknowledge that they might have difficulty fitting in but working with them. It's symbolic, and so I can see why your social worker is querying it.
However.
The cohesiveness of them into your family unit and social context needs to be acknowledged too. For example you could also consider the sibling bond and how it would affect them too- it could be quite othering for the child to have 2 called Sarah and James and then another called, for example, Superman. They might resent it later on, especially if they have issues with being teased or even the burden of having to be extra careful about being found online in comparison to their siblings.
There may be some space for middle names here- there's many a child who uses a middle name as a known as, or alternatively have nods to heritage names in the middle. My nephew has 4 middle names, referencing various grandparents, which some may describe as overkill until I remember that ours have 2 middle names incorporating their originals, and a double barrel surname. None of them go together, or really make sense as a cohesive set of names, and there are loads of them, but who is counting? Loads of kids have go by names, and it's easier to say oh I go by my middle name, than I go by a name I don't even have.
There is also the potential nicknames- Loki could be shortened or lengthened into a cute family name. I initially thought my kids names would be shortened to one thing (we had it planned and everything), but as it happens I apparently call them anything that makes my brain happy and that sticks at the moment. Loki could be Loks, Kiki, Ky, Ki, Lolo, Lola, Lokiboy, Li, Liam. One might stick. But also it might not, and you would have to be at peace with that.
There's lots of books you can get for kids who have unusual names to support them (my name is a song comes to mind). And I would be more inclined to think that the most important thing (barring safety), would be to demonstrate to your child that you accepted all parts of them, including the name they were given, but acknowledge they might feel differently and want to change it later on, which you can support at whatever age by amending by deed poll. We have told ours we love all their names because they are special to who they are and have meanings to their life story but if they ever want to drop or change them, we don't mind. So far they don't give a shit because they are far too busy being a pair of clowns, but one day they might decide they want to drop or change the names we gave them, or that BPs gave them. It will sting if they reject 'our' names, but that's precisely the reason we kept their original middle names in. We didn't want them to feel the sting that we didn't like their names.
Finally, you also do just get used to a name. It's amazing how sometimes things just sort of become the norm. Sometimes people do just name their kids unusual stuff. I reckon there is many a Danaerys who will be going by Danni in a few years time, or even who just rock the name. Some kids just fit with a quirky name. And if you are in a rural area, realistically how often will they have to explain it? They will grow up with these kids forever and they will honestly just be used to it. Maybe it might be noted once or twice when a substitute teacher comes in, but in general if the only thing that is specifically notable to the local community about your kid is their name, you will be lucky indeed.
On the whole, I'm team shoot em all let God decide- I would keep the name in at least some respect and let the kid decide when they are older, and nature take it's course with whatever nicknames eventually evolve.