If we look at the blood, than formally one can hesitate. But the main thing is an upbringing. Russian grand dukes were reared in Russian traditions. They since their childhood knew that they are Russians, and their bond with foreign rulers means nothing if it opposites interests of Russian state. Moreover it doesn't matter how pure Slavic was their blood, because their belonging to Russian family is indisputable. Slavic families are determinated through a father. If your father is Russian, than in principle your are Russian. To elaborate on this question one should also analyze blood compound of Prussian princes. Many of them are likely to have the blood of Pomeranian and Polabian slavs, who I consider to be Russian ancestors in Novgorod Republic.
I will not make any further comments since I do not want to provoke another Battleground-thread. I just want to add that it does not mean anything that many Russians want the Empire to return, many Russians also want the Soviet Union to return since Muscovians always want to have a great Empire were they can gain profit from the suffering of others without having to work themselves.
No. Those who want to return Soviet Union are Soviets, not Russians. The difference between Soviets and Russians is huge. It is an essentially different world-view, culture, traditions.
I just tried to explain in this post why Russians want a return of the Russian Empire. The lands under Russian rule of that time don't disturb us. Why do we need eventually all these Azerbaijan, Turkmenistan and so on? Even Ukraine (except Krim), Poland and Finland don't signify anything for Russia. We have a lot of our lands to settle in there and need to keep these lands for us, not give it to Chinese as a present. This is a real problem. But coming back to the question. Russians want the Russian Empire because it was a real Russian country as I said, where Russian were at the top of the social pyramid and Russian folk was the main host of the country. The elite of that time was fully congenial to the Russian soul and could be treated as its essence. These people were annihilated during the Civil War.