1. There is no general attitude of submission. As you say, nobody will ban octoberfest because it offends muslims. And again "Ehe für Alle" demonstrates that Germany doesn't give in to all aspect of "muslim culture". In this example the homophobic behaviour that is common in a lot of muslim countries.
2. I'm ambivalent about diversity. Because it gets thrown around in many debates about environmental politics. But I don't see your point. From the viewpoint of "race-realists" race-mixing shouldn't create a mass of similar looking people but a spectrum between extremes(races). E.g from the races white and dark you would get white, toned, brown dark. So all in all race-mixing would expand divercity.
3. I think we're missing an important aspect here. That is that the new erect building should be a place for an "open dialogue" a "museum [...] for the whole world community". Which means that the place should try to be all inclusive. From that standpoint it does make sense to not put a christian cross at the top of the building.
If they were to build a christian musuem or church then I would concede to you that you might have a point. Not one I would immediately agree on, but one that should raise public discussion.>Why are Europeans not allowed to be
We can and should be. The thing is that normally we don't see it that way because it is the status quo. I don't see a lot of people who are ashamed of themselves or that people get shamed for celebrating your national holiday.
There is not a lot of shame involved in having an interest in Vikings.
What people get (rightfully) shamed for, is celebrating amoral and unethical historical figures.