IMHO, the way to make a semi-feudal society work for DnD is to make the nobles pretty tough.
IIRC, medieval knights were a martial group, training quite a bit. They had superior arms and armor compared to other groups. While they weren't exactly super rich, they were richer than most others. It's not until later (high middle ages?) that merchants rise to power.
Now let's translate that to DnD. We have a group of people with good training, and lots of hands on experience. That means that the group will have a high average level. Also, DnD has classes based around almost every stat - even lords that would be crappy fighting in plate might have other high end skills, like wizardry. They have good equipment - which means that it's going to be magical. Because magical equipment isn't going to wear out like normal stuff, it will be passed down in the family until lost. Since it's sticking around awhile, in times of prosperity, the nobles can probably afford to have it upgraded.
There will probably be a church or group of churches allied with the nobles too. Siamorphe and Nobanion are both allied with nobility in FR, and many of the gods would be in favor of the status quo. While everyone may have access to healing magic through goodly religions, nobles, because of their strong affiliations with some churches and the fact that they have money, will have superior access. They might even recieve ressurection type magics.
Now we have the favored scions of noble families having excellent training, inherited magical gear, institutional support, and favored access to healing and possibly resurrection magic. In order to proactively eliminate threats to their realms, they'll probably undertake some adventures too. This may create a crowding out effect as noble and noble backed groups clear out areas or solve problems that would otherwise be done by independent groups. Of course, they might think of things the otehr way around - "this is our dungeon, peasant." Because of their advantages in equipment and because they can afford raise spells more easily, adventuring is a much lower risk proposition.
Additionally, the nobles will probably have arrangements with other power groups. It's not like that band of adventurers will be the first people to ever think of attacking someone magically. Do you think most wizards want to be hated because the actions of the terrorist planar binding guy? They have to prep spells in advance and need to sleep, so they'll probably want to be perceived as neutral in most matters. It might be hard to do spell research with a mob outside. Spellcasters might want to police their own. Groups of wizards might trade magical services like wardings, the occasion spell, and or protection from other magi for exceptions from taxation, military service, having some sort of special quasi-noble status, etc. Also, wizards are another class that can work well with inherited stuff. By inheriting a bunch of spell books, a noble wizard might have edge over an independent one - and copying rights could be used as bargining chip to gain the services of other wizards.
Once you integrate nobility into the magical world, they don't look nearly so helpless. Of course, I think there will be some important differences caused by the presence of powerful people, but it doesn't necessarily mean that any given social structure is doomed.
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Oh yeah, I forgot to mention that any social structure could be attacked with terror spell assualts. So if the democratic adventurers overthrow the kingdom, their holdings might then be assualted by escaped royalist backers. I can democratic style governments folding more easily to that kind of external attack than oligarchies.
