To recap, my current understaning of the thought experiment is this:
Premise 1: Evil is harming, killing, or oppressing others.
Premise 2: Good is selflessly helping others.
Premise 3: It's possible for Good to win with no harmful consequences.
Premise 4: If Good wins, Evil acts are prohibited, but everyone is otherwise free to pursue their personal goals in a manner of their own choosing.
Question: What reasonable motivation would Neutral have to prevent Good from winning?
Going by these, I can see many Neutrals opposing Good. Say the Neutral lord owns a well. The neighbouring Good lord says "hey, I am altruist, so when I see my peasants dying from starvation because of a draught, I look at my neighbours for help, because I would help them if the reverse was true". So he goes to the Neutral Lord and says "my people are dying". The neutral lord says "Tough luck for your peasants. It must really suck to be them. I am not evil, but i have absolutely no interest in sharing my harvest, you should have build a well to irrigate beforehand, like I did by forcing my peasants to work on the construction of this public buillding. I will do no evil, and I will eat my food with a profound empathy for your dying peasants, yet don't bother to do anything, even if it allows Lord Evil to invade your land while your starving peasants can't defend themselves from his well-fed armies next year". And this same Neutral Lord would side with his Good neighbour if the advance of the army of the Evil Lord into the Good land would become a threat to the Neutral Lord. So he can be helping both sides depending on the situation.
Though it's less Muscular Neutral than Uncaring Neutral in this particular example, contrary to the one where Muscular Neutural actively battle the force of Good because they simply don't believe them when they say "don't worry, we're nice people, you'll be allowed to pursue your personal goals unimpeded when we will have triumphed over Evil". After all, the Evil side is certainly making the exact same promise to the Neutrals if they side with Evil to wipe out Good.
A more muscular approach could be Lord Neutral attacking whichever of Lord Good and Lord Evil to ensure they live on to reignite the fight at some point later, so they ensure that their two powerful neighbours keep neutralizing themselves over ethical concerns while they can merrily mind their own business. You might want to know why Lord Neutral wouldn't be happy with a twice-as-large-land-of-Lord-Good? Who would he be selling weapons and healing potions to?
And it would be very dangerous if he were to take an action that would be considered Evil by his neighbour, even if, being Neutral, he can estimate that a little oppressing is good (for example, confiscating wealth to build a school). Who knows if the Good neighbour will focus on seeing oppression (something that they feel justified to act against) in this course of action instead of seeing the benefit of creating an educated elite? It's better if they are busy fighting a children-eating devil worshipping Lord Evil, so, to ensure a wide range of possibilities in the future, it's better to have both side fight each other forever, so not to close the possibility of being slightly evil at some point in the future if it suits Lord Neutral's policies. Being neutral, they can be good one day and evil the next day, according to circumstances, so they shouldn't be placated by an outcome where there is no harmful outcome to non-evil people...