Just a few ideas - I am not comfortable with the strength of the guilds on Harn/Lythia - and have thought of ways to reduce their strength. They are far more powerful than Terran guilds ever were, and seem a hindrance in some ways on Harnic economy.
One thought is to make them have some influence in cities - but little or no influence in rural areas.
Also, I think certain guilds would have some power still - those that are predominantly "urban" in nature.
Others may have little power or be non existant - particularily if a predominantly rural industry.
I agree that Guilds within Harn/Lythia are more powerful than they otherwise might be, and ever since I first read about them, I've looked at ways to changing them.
The first thing to understand is that attempting to enforce some guild privilege, much like any protectionist scheme is incredibly expensive. You have to pay people to find those who are violating your privileges, which while more efficient in larger population centers, would be a losing proposition in rural areas where the people are spread out.
In the long run in a true Free Market environment while protectionist systems may rise up, it won't take them long to collapse. The only way such a scheme can perpetuate in the long run is if the cost of enforcement of such scheme was offloaded to someone else, such as the state and inevitably the tax-payer.
To take an example, slavery in the past was only ever economically viable because the state paid for the slave catchers. If your slaves ran away, all you needed to do was contact the local slave catchers who would do the work for you. Without state slave catchers, a slave owner would need to pay out of his own pocket for every man who worked for him to ensure the slaves wouldn't run away. To use an real-world example, slavery in I believe Brazil practically ended overnight when the state announced that they would no longer be catching runaways.
What I can see occurring is that representatives of the Mangai who wanted to establish guild privileges in an area would, much like it is done nowadays, is for Mangai representatives to "wine and dine" and/or bribe those who have the power to tax the locals which would enable the funding of such protectionist measures.
In larger population centers, the Mangai could rouse the greed in the various tradesmen in enticing them that such protectionist schemes are for their benefit, in addition to bribing the Lord or town council. The Lord(s) sees that this is what a vocal minority of the people want, and with enough pressure from the Mangai and tradesmen, such schemes would get passed. The cost of such scheme would be easier to hide (spread out across more people) that few may not notice for a time that their taxes are rising. If those who benefit see their income double or triple, they may not care if their taxes go up a fraction of what they were.
In rural areas where the manors are small, and there's few tradesmen who would benefit from such protections I imagine that the Mangai would see it as a losing proposition. Considering that in the feudal system there are contracts which spell out the feudal obligations of lord and serf, the increased expense would fundamentally fall to those who don't have such feudal contacts. A whole lot easier when there's more people to spread the costs of enforcement to. A close-knit community would resist such attempts, unless there was some sort of economic warfare at work where nearby guilded towns prevented people from non-guilded manors from selling what they produced in the town.