It is an interesting theory, however I doubt it would be likely in-canon due to the simply fact the Germanic versions of the gods are identified as identical to their Germanic counterpart. It is very clear with Mr. Wednesday, who is Odin and refers a lot to Nordic mythology, yet is also frequently called Wotan by other gods, clearly showing the two versions of the character are a same god known by two names.

In the novel there is no "Donar", just Thor. The television series decided to add Donar to his name (even though if I recall he is also referred to as Thor as well as Donar? I need to check), but it seems to be in the same logic as Odin/Wotan for Mr. Wednesday. Two names for different people, but the same entity.

In real-life culture, while I am not sure of it, I do believe and recall that the Germanic versions of the Nordic gods were actually very similar to the Norse versions, to the point that most books only see them different in name and in a few little details about their legends. Though again, I am not sure and the difference might be just more than a new name.

However it is possible that there is a version of Donar existing somewhere in Germany separate from the Thor of the Nordic countries. It is quite possible.

Thanks for the answer.

It was a pleasure to share my thoughts here.