This is going to be my last post on this thread because I think that we're going in circles here at this point. To reiterate, I don't see anything wrong with trading links between the two, but substantial settlements will require a lot of resources to continuously maintain, which will be extremely difficult to manage in such an isolated environment.
No i am pointing out its an island and in my post the troops went inland to the south east found the Poronay river first after being attacked by Nivkhs. They learn of the Poronay valley by travelling down it. Theyare already in the far north of the island the only way to go is south. Ships can easily scout the coast and would be sent to find native settlements seeing much of the coast.
They trade along the west coast with fishing villages and learn about the south before the Fall of Balhae the time between Seon and the Fall is when all this happens which is over 100 hundred years. The forts are establish slowly and more forts appeared in the better regions because farming could be practice there and those goods needed protecting. The trading was limited but only small forces were needed to suppress the native forces so only about 500 troops were stationed between all the forts on the island.
When the Balhaeans fled to the Island they would have already known about the southern region. When they go south they would encounter more resistance because they would need more of the resources to establish themselves. Since there would already be some troops on the island and more would come with the fleeing group its not impossible to see them field about 1200 if they recruit natives and train whoever they can for defence. Forts are a hard thing to defeat without seigewarfare experience. The natives are not likely to win.
Again, the longer the process, the higher the possibility of abandonment. Not all rulers are keen on supporting their predecessor's policies, while the ruler and the court would not be willing to show support if native raiding parties continue to destabilize the area. In addition, it will require a significant amount of resources to build even one fortress, along with the fact that the soldiers cannot be high in proportion to the total amount of settlers for a stable settlement to be established. If the migrants somehow manage to number around 1,000 before the state's downfall, which in itself is probably highly unlikely, around 50-200 of them will be soldiers, and 400-600 will be farmers, hunters, or traders, so that leaves only a few hundred at most for builders, as half the population would be theoretically composed of women.
With such a small population, it will probably take a decade to build just one fort, not to mention the required costs for expanding the structure, along with rebuilding and/or renovation. Those who attempt to explore the area for more resources in order to expand/contribute to the fort and/or settlement can be easily ambushed due to low numbers, and either structure can easily be attacked before preparations are complete, at which point the costs are much higher than the benefits. Although I certainly realize that several forts in a row can serve as a firm bulwark against potential invasions, such as the ones used by Goguryeo against the Sui and Tang, the relatively limited amount of resources and manpower that can be utilized within a limited area in Sakhalin means that the two situations are not comparable.
The realative small military force needed to maintain the islands made it possible but they would still use this area to send mostly troublesome officials and troops who lost favor of the court. Seeking to regain favor or make a profit they would seek success at all cost winning enemies and allies among the native peoples. Some alliances made control easier as well as i pointed out there were people natives who benefited from their arrival.
This is why there wasn't more then 1000 on the island pre fall but there was some gains to be made selling native produced goods. The natives brought the goods to the ports to trade. Sea trade was more prevelant even before the Fall as they did not have to travel north overland constantly.
it didn't do much more than bring more Balhae officials and those familiar with the culture to the island. But more families helped for growth. With the heavy Ainu population the chance of absorbtion was lessen with a second influx.
I wonder how Balhae language looks as the Ainu are assimilated?
Those who have lost the favor of the court can just be sent further north in Manchuria, as the climate would be much harsher in that region, and it wouldn't make sense for them to be exiled to established close-knit settlements. As I stated earlier, any goods that might be produced by such a low population would probably be mostly used by the settlers on the island, leaving very little to ship back. In addition, considering that the mortality rate will probably be high within the first few years/decades, a few hundred would probably only boost the population to its former levels. In terms of the languages, virtually nothing is known about the one that was spoken in Balhae, not to mention the one spoken by the natives around that time period, as the language would have evolved for centuries, so I don't think that anything significant can be deduced.
That is the idea to put as many men on the walls as possible if a major native force arose. But as that is unlikely any way its fairly moot. That 500 force would be brought with the fleeing force to join the soldiers already on the island. Would then defend invasion from the north and settle in the south.
As to settlements as the forts were likely undermanned because more men wasn't neccessary they should be able to support a good number more. Then a area of Balhaean native contention might be the fleeing Balhaeans taking over villages and settlements of the Natives by force.
The combined Balhaean population of all those already there and those that fled should be 6000 and the native population 10,000. Since Lesser Balhae will be in the South the number of natives they end up fighting will be more like 6, 000 as 2,000 are pro Balhae.
I would give the range for the second significant wave of migrants to be around 1,000-5,000, with the actual number closer to 2,500. Larger numbers potentially mean higher mortality rates, as the newcomers need to adjust, and there are not enough resources within the area to sustain such a high population. In addition, I already provided the potential ratio of soldiers to the rest of the population several times beforehand, so the combined total number of soldiers after the second migration would be around 500 at most, which wouldn't exactly be ideal for repelling a numerically superior force. As I stated earlier, the natives were vastly outnumbered when resisting the Japanese for so long, so considering the limited settlement(s), the area would be quickly destabilized by numerous raids soon after the second wave.