I have thought a lot of why König Wilhelm II. was only against a Princess Pauline-Romanow marriage.
First comes to mind that Wilhelm II. was a convinced Lutheran, he named his only son Ulrich after Duke Ulrich of Württemberg (1487–1550), the first Protestant ruler of Württemberg.
But when he wanted Pauline to marry only a protestant, he would have excluded all non-protestants in the instruction, such as the Kingdoms of Saxony and Bavaria.
Second: Pauline was heir to her father. When the König does not want her to marry outside the country, and keep the fortune in the German Empire, he would have mentioned that in the document. He did not. No other foreign court was mentioned, so it seems the only one he don't like was the russian royalty.
Maybe König Wilhelm II. knew something about the Romanows we don't know?
Or it was simply that he wouldn't let his only child go to danger, you know, the were attempts to kill the imperial family.
Wilhelms II. first wife Marie died in 1882, their son Ulrich in 1880. Königin Olga Nikolajewna and Herzogin Wera Konstantinowna were both alive and at the württermbergian court at this time.
I think Wilhelm never talked about Olga, but of Wera he said: "She is ugly but not without heart".
That sounds he don't liked her, and in Paulines (later Fürstin zu Wied) memoirs "Vom Leben gelernt" she writes that she barely knew Wera's twins Olga and Elsa and they were never close.
Pauline was born in 1877 and Wera's twin-girls in 1876, it's curious that they were so close in age and from the same family and didn't see each other often...