Even if they are from the sixteenth century, it would still be something of a mystery. The voyage of the Russian ship St. Gabriel in 1732 (~200 years later) is generally believed to be the first time Europeans reached Alaska. The first Spanish expeditions to Alaska didn't occur until the late 1700s. Also, the suggested geography depicts the Aleutian Islands, and I'm pretty sure the Spanish never made it that far. The Russians were moving into the area in force by that time period -- the approximate boundary between nominal Spanish territory and nominal Russian territory was around Sonoma County, California. And then the British came in...
It's certainly not beyond the realm of possibility that Chinese traders (or perhaps Chinese trading partners) had ranged that far. Certainly a lot of the furs that the Russians and British took out of the region wound up being trafficked into China.
It is indeed possible that Chinese trading vessels may have reached the Kamchatka peninsula and the Aleutian Islands. The more unlikely part here is that those voyages would have been made known to a random Venetian barbarian who happened to be passing through, and that said Venetian would be able to obtain such detailed maps. If I were going to come up with a hypothetical explanation for the maps beyond 20th century forgery, I'd say that it's more likely that some 16th century Manila galleon strayed a bit north on the return trip to Acapulco and did some secret mapping. The Spanish Manila route actually got surprisingly close to Alaska: http://historicphilippines.com/wp-content/uploads/2014/05/ru...
That's a stretch too, of course. But the Spanish were extremely secretive about their cartographic knowledge so the mere fact that there was no official announcement of a discovery doesn't necessarily mean they didn't know about it.
Polo describes (without claiming to have been there himself) a "Land of Darkness", rich in furs, which borders on "Great Rosia", the people of "Rosia" being Christians of the Greek (i.e., Orthodox) form.
What you imply is correct. The Russian conquest of Siberia was basically enabled by the decline of Mongol influence, which necessarily post-dates Marco Polo's travels.
Dating the sheepskin vellum to the sixteenth century just means that the vellum is from that time. It doesn't mean the ink is from the same era.
The text that was on the vellum might have been scrapped off, perhaps simply to reuse the vellum (making it a palimpsest), or as a deliberately forgery on older material.
It's certainly not beyond the realm of possibility that Chinese traders (or perhaps Chinese trading partners) had ranged that far. Certainly a lot of the furs that the Russians and British took out of the region wound up being trafficked into China.