Here's a random thought that popped into my head while playing this - after a certain point, what's stopping the MC from just taking over Arenfield?
You eventually form a private militia that outnumbers Girion's that are tasked with guarding Arenfield, the mines, and the trade route to Arenfield. What's preventing the MC from just waltzing in, saying "hey, I'm the baron now, I have a bigger army than you, GTFO"
You might say that Girion has political connections with King Eric. If Eric hears Girion die or is forcefully removed or imprisoned, he may end up sending his army there believing bandits took over or something or think that the MC isn't a trustworthy or reliable guy to oversee the land. But to refute that, I can argue that the MC is far more valuable to the king than Girion for multiple reasons.
1. The MC can train, arm, and rally troops, as well as maintain their morale by paying them which Girion seems incapable of doing. If this is a feudalistic society (which it probably isn't) then this is invaluable to Eric because he needs vassals to pledge troops to him. Even if it isn't a feudalistic society this is still important to maintain law and order, and having a private militia as a potential ally still doesn't hurt.
2. The MC is the reason why the mines are functional, because again, he has his private militia securing it. Without security the mines will just get ransacked by goblins like what happened when Girion was in control. This is important to King Eric because Arenfield needs iron to supply his army with arms and armor.
3. The MC is the only reason why the trade routes are secure, again, for the same reason above. This will improve the economy of Arenfield as more merchants and travelers are encouraged to pass through there since it is safer. They will carry gold with them, use the tavern, and buy things which will improve the economy. Better economy = more revenue for the king
4. The MC is the only reason why Arenfield is secure. If the war ends up on Arenfield's doorstep the MC has all of the leverage, he is the one who decides to keep military around to defend the place.
5. As mentioned above, all of the benefits of MC's private militia can be leveraged. MC can threaten to take away his soldiers from the mines, or have them stop guarding the trade routes, or have them stop guarding Arenfield, and that gives him a lot of influence in negotiations.
6. The MC, over time, develops a lot of relationships with the villagers and they end up trusting him because he seems competent and capable of fixing problems. He eventually gets a good reputation. In contrast most of the villagers seem to greatly dislike Girion and some of them even think he poisoned his predecessor. Having the MC in charge reduces the odds of a revolt since many of the villagers being to trust him.
7. It's implied that the reason why the taxes are so high in Arenfield is not only King Eric's war, but that Girion is simply greedy and wants to maintain a lavish lifestyle. The MC does not need to live like Girion to survive, plus has alternate streams of revenue, like the fur trade in Umah. Combined with improved trade after the secured routes he can afford to cut taxes while maintaining Eric's quota, which would again improve the economy as the villagers have more disposable income, or offer Eric slightly more as a sign of good faith. Having gratuitously high taxes, to the point where some of the people who've lived there all their lives may need to resort to begging, is not good if you want to maintain social stability and again, can easily lead to a revolt.
8. The MC can also gain political connections by gaining favor with the Queen, Helena. She can easily just put a good word in for the MC and say Girion was incompetent.
9. The MC also has connections with the town guard through Lyvia, the Captain. Lyvia can simply order the guards to stand down and it's clear she likes the MC far more than Girion. The only obstacle to this is if Lyvia values her oath to Girion (ie she dislikes Girion but she swore an oath so by principle she must uphold it) or views the act of a hostile takeover as unlawful and values upholding the law over all else. Even if this is the case, there is a solution to this (see below)
10. Arenfield, as well as Girion's mansion, do not seem particularly well fortified. Outside of the guards, there's only a river, and even then there are two bridges that can be used to cross it. Plus there's the secret entrance.
11. MC is literate. This is makes him a good taxman and contractor, which is a very nice bonus if you're applying to be the subject of a king
Now you might say despite all of the benefits of having the MC in charge, King Eric might not care - he's a huge "Law and Order" guy and believes in upholding the rule of law, whether for pragmatic reasons or idealistic ones. This I doubt, since he seems to have no problem with wanting to arrest and execute Ophelia after the envelope shenanigans with no trial or investigation, and according to Thomas it's solely to have a scapegoat to raise troop morale. This implies he believes the war effort > rule of law and having the MC in charge is far more beneficial for him to fund and maintain his war, for various reasons. But even if this is true, there is an alternate route the MC could take.
The Legal Route
MC can simply hold a democratic election to oust Girion. This is presumably something that can be done and has been done before, there were elections in Arenfield in the past and they just mysteriously stopped for no apparent reason. This is an election again that the MC can easily win, as he gains the favor and trust of numerous villagers over time and many of them don't seem to like Girion very much anyways. If Girion pulls a Trump and says the election results are illegitimate, there is nothing he can realistically do with the MC's military connections (Lyvia can just order the guards to stand down) and with the MC having a larger private militia. Girion also cannot prevent an election from happening because again, he lacks the military capacity to enforce that.
If King Eric does not believe the election results, they are very easy to confirm - Arenfield is small, he can simply ask the villagers themselves if they voted for the MC. In the end, Girion does not actually provide anything to Arenfield aside from just being a middleman to royalty and the taxman, which the MC can easily fit the role of, especially since he's literate, so why would any of the villagers honestly want to keep him around?
Hurdles
There are honestly only two major hurdles I can think of when it comes to MC taking over Arenfield.
1. Girion is a very, very close friend to King Eric.
Like literal blood brothers or something. Or even secret gay lovers. If King Eric wants Girion alive or in charge entirely because of sentimental reasons, and he's such close friends with Girion that he wants to keep him around even if it undermines his own war efforts then that can cause a lot of problems if a hostile takeover leads to violence (Lyvia doesn't tell guards to stand down, in the chaos of battle Girion gets killed). This can be mitigated if MC simply imprisons Girion, forces him to flee, or again, holds a legitimate election against him. Even then though, I doubt this is the case, not only because it seems Girion is more intimidated by Eric than anything else but also because if they're really bros, why doesn't Eric bother sending any men to help protect Arenfield and Girion in the first place?
2. The Church
I don't know much about how the church works, but if there's some way for the MC to get declared a heretic or a demonspawn or an unholy abomination who will lead everyone to Hell then that can cause problems too if everyone is either religious, or sees the church as a legitimate religious authority. If MC's own men are religious that's an even bigger problem since they may end up abandoning him. This I question, since we barely ever see anyone ever going to church at all, but it's still a possibility if MC goes the violent route, kills Girion or his men, and then Pater Francis either declares he's evil, or sends a message to some central church (ie like the in-game equivalent to the Roman Catholic Church) to send an inquisitor to do an investigation and declare him a heretic.
This, however, can be mitigated if MC goes the legal democratic route (if Francis like Girion more than you, how is he going to declare you a hellspawn simply for winning a legitimate election, and if Francis like you more than Girion, he may even vote for you) or if Francis is actually corrupt. If Francis is corrupt then he can be convinced that MC is better than Girion because with MC in charge more gold will flow to Arenfield and eventually the church and more travelers will end up going to the church.
After all of this you might say that the MC simply lacks ambition or the will to take charge of Arenfield. But again, this is something I doubt. MC became the chieftain of Umah, and took charge of his own private militia. He regularly fixes problems in Arenfield too. He clearly has some kind of ambition or drive, otherwise he wouldn't have taken the initiative to do any of these things in the first place.
So with all that said and done, why even bother keeping Girion around? Why not kick him out?