I don't think I've ever heard Peter the Great referred to as not quite "stable", but OK.
He was suffering from <forget the name of disease> that makes mental concentration difficult and "favors" physical activities.
Right, and as they declined in Germany, the Hansa becomes less relevant, meaning that Lord Novgorod may need to start sending out their own merchants to maintain what was already there.
An idea not being supported by anything besides wishful thinking. There was no track record of the Novgorodian wish to start a naval trade. They did not even built any port facilities on a part of the Baltic coast they owned. They did not have any shipbuilding experience, etc.
The idea of a Republican, more Mercantile Russia isn't off the table with a Novgorod-centred Russia rather than a Moscovite one
Just keep repeating the same thing based strictly upon an assumption that it is "logical" does not make it more realistic. Medieval Rus was not "republican" and could not become one: too many factors were going against this. With the exception of Novgorod and Pskov, the Russian states had "standard" feudal structures and their social development was going along the same lines (with the adjustments to the regional specifics) as in most of the rest of Europe.
How militarily weak Novgorod would manage to subdue the rest of the Russian states? This could be (and had been) done by a military force which Novgorod was lacking and by a consistent multi-generational policies which Princes of Moscow and Lithuania had and which Novgorod with its unstable governments simply could not have.
Right, so the capital of the Rus' people prior to Kiev, that is centuries older than Moscovy, which itself started off as a trade outpost in the 13th century cannot possibly become the leading Russian Principality.
Kiev was not a capital of the "republic" so analogy does not make a slightest sense.
Right, that doesn't sound like drivel at all. Moscovy can't be repeatedly burned to the ground by rivals, like it was in 1238, nope. Totally inevitable.
Raise of the Moscow as a specific center of the Russian unification was anything but inevitable and for quite a while its princes had been getting their seniority by acquiring title of "Grand Prince of Vladimir". With the same success it could be any other princedom of the Vladimir-Suzdal Rus.
You want another PoD that means that Moscovy doesn't rise, and allows Novgorod to be the leading Russian Principality? Novgorod forms an alliance with Uzbeg Khan rather than Moscovy, or Tver does, or any of the other Russian cities. Novgorod hires mercenaries to secure Moscovy when it is weaker, bolstering its trade networks, and ensuring a stable Russia for the Golden Horde which then means Novgorod is seen as a shelter during the Mongol Period.
Sight. You really don't understand the basics. "Novgorod" could not became a Grand Prince of Vladimir (the title goes to the person not a state) and rulers of the Golden Horde had been traditionalists enough not to introduce the new and confusing schema. The title could go to a prince of Ryazan or Twer and it is an open question for how long it would be held by the rulers of a specific princedom and which city will end up as a historic capital of the unified Russian state but, not being even a "principality", Novgorod would not be on the list. BTW, I suspect that by the time of Uzbek the princes of Moscow already had been rich and powerful enough to deal with any realistic competition. Anyway, there is no reason to assume that alternative winning princely line is going to be more "republican" than one of the OTL.
The story about the mercenaries is, indeed interesting but (a) Novgorod hardly had enough money to hire 20 - 30K of the German mercenaries, (b) during the Middle Ages these numbers would not be easily available, (c) there was a high chance that instead of going to war with Moscow (not the next door) these mercenaries would opt for looting Novgorod, (d) the Mongols would not take such an idea lightly because Russian princedoms had been his vassals, etc.
Also, a point on the whole West/East thing.
You can have a PoD pre-1900 that has repercussions AFTER 1900. As such, making sure Russia is so tightly coupled it would be absurd to try and separate it from "The West" would be part of that aim. Heck, a merchant republic that leans heavily on an alliance with Britain to ensure open access to the North Sea ties the two together VERY tightly.
OTL Russia was an acknowledged "European state" (which, with an adjustment to the anachronistic terminology of the thread amounts to the "West") since the early XVIII so if we are talking about the "absurdities", the attempts to claim otherwise belong to that category.
As for the alliance with Britain, OTL Russia was in almost uninterrupted alliance/trade relations with it from 1551 when the
Muscovy Company was formed by
Richard Chancellor,
Sebastian Cabot, Sir
Hugh Willoughby and several London merchants; Ivan IV opened
Arkhangelsk to the Company and granted the Company privilege of trading throughout his reign without paying the standard customs fees.
[36] Muscovy Company retained the monopoly in Russo-English trade until 1698. There was an interruption for the ECW - beheading of the king was not appreciated and also short interruptions during the GNW and post-Tilsit but close diplomatic and economic ties existed all the way to the mid-XIX. The CW was the 1st time when the Russians and Brits had been really fighting each other.