I am not saying that "The Truiet claim the throne". What I am saying is "Nobody gets to be the King without the Truiet agreeing". Because when it comes down to it, it wouldn't be hard for them to let the ships of a bad ruler get destroyed by Pirates while they carefully protect the ships of an ally.

Don't assume people play politics nicely.

If you control the trading, you control the value of commodities and resources. Via the shipping paths (the only credible export path based on this map and their apparent isolation), the Truiet will have the physical (and therefore political) power to basically destroy whichever guild/family or tribe angers them by cutting off their supply of cash.

What makes OPEC powerful? They all agree about the value of their goods and it's hard to get them any other way. They control the shipping of oil. Even if they didn't already control its production.

What made the Dutch East India company powerful? Control of the delivery of goods, and so on.

It isn't the MAKERS of materials that always hold the power, it is often those who TRADE it.

Negotiations over the trade lanes, I would suspect, would follow this kind of format:
Tribe A "Why should the Trueit control the sea lanes?"
Trueit: "Sure, we'll take our ships out of the water and let the pirates get your ships until you can defend them yourself..."
Tribe B: "But we can't afford to lose many cargo ships to pirates, they are already taking enough"
Tribe A: "But we don't want to pay taxes to the Trueits"
Tribe B: "Taxes vs complete losses... we'll take Taxes"
Tribe C: "I agree with (side)"
Tribe D: "We agree with (other side)"
... [insert stalemate here with Trueit retaining the role and power associated with protecting shipping for the whole nation]