To begin with, unless the CoTs you're talking about have empty slots, and excluding the +1 merchant/year from a monopoly (which doesn't stack), a monopoly will only serve as 6 merchants, in addition to exposing yourself to competers, since competers will usually seek competing out the monopoly before competing each other.
So, if getting more than one monopoly will take place in CoTs that do not have empty slots, and will make you have less than 5 merchants in other CoTs that you care about, you might actually benefit more from not getting these extra monopolies. Of course, there's into account the trade good domination thingy and the difference of power between various CoTs, so there's no hard-rule on what's right.
- I never saw the decentralization thing you're talking about.
- The game is designed such that the more merchants you have around the world, the less your compete chance will become, thus the more CoTs you wanna involve yourself in the harder stuff will become.
- It's true that more CoTs that you own, the more merchants you get, but previous point also applies and in some cases more CoTs could prove counter-beneficial.
Four things you need more of: Compete chance, placement chance, trade efficiency, and yearly merchants.
- Compare various types of governments.
- Compare possible state religions.
- See about buildings & national decisions, also national modifiers.
- See about advisors & ruler.
- Avoid infamy & war, max stability & prestige.
- The less nations who can trade in a certain CoT, the easier it is for you to control.