I just found something about this! Behold!
The Legend of the White Cowl...
It claims a great relic-cowl was once in Rome, and was sent to Constantinople, and then, when the Turks were nearing, it was then sent to the Archbishop of Novgorod. This would explain (even if this legend is false) why the mitre might have looked like the western type of mitre, in order to re-enforce the legend of it coming from Rome originally. It also fits with the white pallium, which all Catholic bishops were to receive from the Pope.
Now, wonder when it first stopped being used... hmmm
No, the white cowl refers to a white koukoulion, such as the Patriarch of Moscow wears. This legend is the basis of the Third Rome theory. Interestingly, since it assumes the Donation of Constantine, anyone who subscribes to Third Rome must also accept the Donation of Constantine.
Yes, one could say it refers to the koukoulion, but based on the account of this witness the koukoulion was not being worn by the Archbishop of Novgorod at the time of the visit. The Archbishop of Novgorod:
"but the bishop of Novogorod alone wears a white two-horned mitre after our fashion" (this is written from the prospective of a Catholic)
"The bishop of Novgorod wears a white pallium"
It is not the koukoulion, at least not at the time of the visit by Baron Siegmund in the early 16th century. The writer clearly describes the other bishops wearing a round black mitre, which agrees with the historical development so he does know what he is talking about.
So, the thread here is looking for evidence as to why the Archbishop of Novgorod would wear such vestments. Based on this legend it gives a reason why the Archbishop of Novgorod had a white two horned mitre which is mimicking the western mitre, and further the pallium to reinforce the legend further. It does not matter if the legend is true or not. The legend simply gives the justification for the Archbishop to wear such vestments, even for a simple reason such as an attempt to increase the prestige of his office even if the legend is false.