Filianism/Déanism is a monotheistic faith. The Nordic faiths (Ásatrú, Vanatru, and Rökkatru) are polytheistic. I have been a polytheist from the beginning, even way back when I was a practicing Wiccan in the early-mid 90s. Some would think that the crossing of polytheistic thought with monotheist belief systems would be some kind of massive 405 Method Not Allowed error. (Yes, I am married to a programmer and I've absorbed some of his arcane language through osmosis. It was not pleasant. I suspect this is what initiates into the Borg collective experience.)
I am clearly an outlier within the Filianic/Déanic community. I don't pretend to be the voice of orthodox Filianism. If I have given anyone this impression, I would like to apologize and encourage you to refer to sites such as A Chapel of Our Mother God and the Daughters of Shining Harmony. I also am something of an outlier within the Nordic faith community. It is something that may cause some upset by others but it is just who and what I am.
I believe that all gods are distinct entities. I believe that there is overlap on what the gods can do because the universe is so vast and complex that the overlap is going to happen to the benefit of the universe. Many hands make light work, as my late grandfather would say. I also believe that every religion on the face of Earth is valid. I do not believe, however, that anyone has the right to foist their faith on to some one else. It is my understanding that faiths that encourage spreading the word of their beliefs are supposed to do so by example and demonstrating their values through action rather then coercive force. Furthermore, it is my belief that all these different religions can coexist peaceably because sectarian warfare is something that humans do.
Do the gods have their wars? Perhaps. The Aesir-Vanir war is mentioned and figures strongly within the mythos of Ásatrú and Vanatru. Thor's battles with Jotuns are well known as well. I am sure in other faith systems there are stories of wars between the gods. I am sure that those wars will happen even in modern times. It does not mean that we humans should wage war on each other just because someone doesn't believe the same thing we do. I digress, let me return to my original point.
At one point, I was something of a soft polytheist. I thought that all gods were different facets of a single entity. I came to the conclusion over the years that this is not correct. At least, not in my experience. I believe it is possible to worship gods from different pantheons. I believe it is possible to have relationships with gods of different pantheons. It is like having dealings with people of different families. They are different from each other but have common traits, in this case of being deities. Some deities do not get along well with others. I wouldn't hail Thor and an ice Jotun at the same time because there is going to be friction between them. (Gods, is that a huge understatement or what?)
It is my experience that Dea is willing and happy to be hailed and honored alongside other deities. It is my experience that she prefers peace and harmony over the struggle to be first in line to be hailed or the only one to be hailed. As she is not a threat to the Nordic deities, they have peaceable dealings with her. I think it is human insecurity projected on to deities when we say that they must declare other religions to be false. From my experience, as long as their people and the pantheon are relatively safe and not restricted in their doings, most gods don't care about other pantheons. Indeed, some will develop friendships and alliances across pantheon lines.
I think this is reflected in the different gods being viewed as the 'same' across different regions. A fine example is Herne and Freyr. I don't know if their friendly relationship was present in the past. I suspect things were more rocky between them when the cultures of their peoples were in regular warfare with each other. Now, however, they work together to the benefit of their followers when invoked together. It is my experience that these two deities regard each other as equals and respect each other highly. And I think this is reflected in how the Celtic oriented modern pagans generally get on well with the Nordic faith oriented modern pagans.
All of this is my personal gnosis but it seems to be paralleled by the experiences of others. It may seem contradictory to worship a goddess of a monotheistic faith and be a polytheist but it is a conflict that arises when one decides that there is only one true faith, only one deity. I know with all the surety of the bones within my body that there are more gods then we can count. They reveal themselves to us in a manner that we might best understand them. All religions are built of a consensus of personal gnosis along different themes. At one point, there was a bunch of individual believers stumbling around in the proverbial dark. Then they discovered that others had the same experiences as them. Thus the accepted beliefs began to arise and what we recognize as religion appeared on the scene.
Religion is something from humanity. The gods give us information and direction. It is we who implement it and act on it with our best judgment.