The bulk of dutch trade was within Europe itself. The VOC offered the biggest profitmargin (if the journey was succesfull) but trade with non European areas comprised only about 10% of the total trade volume. 50 % of the trade was with the baltic sea area, grain and wood. The northern provinces were already increasing their competion there from the 15th century. You can see this in agriculture, where they were switching from grain to commercial products. And this is IMO the main reason for the naval success. The south (Antwerp) was the main trade and financial center and the staplemarket, but they did let others do the transporting job. They relied more and more on northern shipping. The interesting thing is that this developement was independent of the political developments. In a way the northern had to do this. Their clay land which they had reclaimed in the high middle ages, was losing it's fertility, cause the land dried out slowly. The grain produced was not enough anymore to feed the population. This is why they started to trade with Polish nobles, who were just increasing their grain surplusses. Before long they were entirely dependent on this trade, and it remained the most important trade of the Republic during it's existence.
The Dutch benefited the most by having free trade in Europe and therefore the dutch were top dog as long as the two main competitors, France and England were more busy with internal struggles than implementing mercantilistic measures. As soon as those two created more internal stability, they started to overshadow the republic.
There would be an English Civil War, it did not depend on what happened in the Low Countries. The TTL Netherlands would most likely lose all of their colonies in a war with the English Commonwealth. And if Restoration still occurs, since the Stuarts were full of idiots, there would be a form of Glorious Revolution, and most likely a second Republic would be formed.
One of the main reasons of the success of the glorious revolution was that a foreign nation intervened and settled the kings perogatives issue that had haunted british politics for decades. No intervention, or even worse intervention from France will mean it will take decades more to settle or it will mean a permanent damage to british trade, as this weakens the voice of merchants in politics. England will have much more trouble in it's competition with France.
Sure, it could be more secure, but it would not be a sea power and a colonial expansionist like it was IOTL. There is trade-off. Those damn Austrian -Ottoman wars (zero relevance to Dutch interest) would be very costly, and the Netherlands obviously would be a cash cow.
Zero relevance, no. That depends if in a peacetreaty there will be some exclusive trading clauses. But i can agree that a break between the Netherlands and the Habsburgs can easily occur over the financing of the wars.
I wonder though if with Austrian mercantilistic measurements Dutch european trade dominance could exist in the first place, and if it would exist longer. I'm inclined to say yes to the first question, because the foundations for the success were already laid by the 1550's. They were already heavy outcompeting the Hanze and the Italians. The second question i think no, although i also could see a scenario where you have an English, a French and a Dutch-German trading bloc.
