Since I've obviously not played the game yet, I can only speculate.
I don't think it would be as ridiculous as say... Ulm focusing on naval ideas.
It sounds like going trade focus on Ming would be sub-optimal. Gold is gold, and they seem in a much better position to get income from production and taxes (gold is gold after all). However, there's nothing to say you couldn't focus on all three aspects of income with trade simply being third-place-but-not-insubstantial-source.
Even though your node only has outflow, building up the value of trade power is not useless. Instead of focusing on making trade come in, your power makes sure that trade does not go
out. A trade-focused Ming is going to be able to retain the node's value a lot more than one that ignores it, much to the frustration of Western powers who were hoping to divert that value to their old lands.
I assume that you can't just stack all your merchants into a single node, so what to do with the others? Probably best to just plant them into the nodes that give you the biggest return when you collect. Yes the other nodes don't give as optimal a return as the capital one, but income is income, and using your power to divert from one node to another is pretty much pointless. Build your power in the best nodes you can with ships and conquest and take your piece of the pie.
Moving your capital is an option, but how much that makes sense depends on the situation. After all, if you've built up and are holding a near-monopoly in your starting node, it still might be worth more to go with the strategy to retain value at home and accept the 50% hit on other node returns.
Trade and war are not mutually exclusive. Your trade power depends a lot on your holdings, and you can steal trade power from rivals in peace deals. The trade power you steal can be used to help retain value in nodes and to take a larger share of income in nodes that they hold. I could see this being a very effective way to improve your income without having to deal with the headaches that come with over extension. Of course, not going to war means your resources can be spent to build the infrastructure at home. I find it best to ask yourself (and this applies to all Paradox games) "what is the best way to make my money work for me right now?" Building up infrastructure sometimes only goes so far, if your coffers are overflowing and you have nothing to build, sometimes war is the only way to put that money to work. That being said, building up a nest egg for future investments should not be ignored either.
Ming can only collect from nodes that they have trade power in (I.E. own land). A European nation can use their merchants to PULL trade from nodes they do not own land in, and then collect it further down the stream. So unless you're conquest happy, you'll run out of places to put your merchants quickly.
I just want to point out, that owning land in other trade node/regions is not the only way (nor the most efficient) to get trade power. Having light ships patrol in the region contributes a
significant amount of trade power to a node. This is why trade range is very important, because it opens up which nodes you're allowed to spread your influence in, even if you don't own actual land in the region.
