I've always held that in the beginning the fief may have been for a lifetime or several lifetimes, but over the centuries and custom adapting to the same clan holding certain lands that it has become more of a heredity fief as of 720. With Harn and environs being as clannish as they are, this could also mean that if manor holding knight dies without issue his clan may install another member as the hereditary receipiant of the fief.
I was looking for specific notations in published material, which is why I wondered if there was anything official, or if it was essentially a "P-Harn" Issue (ie varies according to the GM). However, based on what you've written above, I thought I'd pass my thoughts on your ideas, and see if said ideas make sense one way or another
First - if we're dealing with something that has a tradition behind it (ie many generations), we're also likely talking about something that tends to be either cultural, or has been established/set in law and followed in that fashion. Keep in mind here, that there are "customs" which are unwritten "laws" and can have effectively the force of law, and then there are written laws that are in fact, laws. Customs can be circumvented (usually with vast disapproval) while laws are effectively NOT supposed to be broken. It is a subtle distinction to be sure, but it is one. One can't really go to court to enforce a custom, while one can go to court to enforce a law. That groundwork having been laid (ie definitions), let's see what the above takes us to.
"I've always held that in the beginning the fief may have been for a lifetime or several lifetimes, but over the centuries and custom adapting to the same clan holding certain lands that it has become more of a heredity fief as of 720"
If you go with the idea that Kaldor came into being around 238 TR (with the combination of the four petty kingdoms into one entity known as Kaldor), then we're dealing with roughly 482 years of history overall. The legal question then becomes one of "with whom does an enfeofing noble have a contract with? Does he have a contract with the immediate Knight that holds a manor, or does he have a contract with the entire clan? Does a "clan" have a legal status that a contract made with a clan member, is binding upon another clan member? Another problem that arises is this:
Does one need to have forebears who were noble, in order to be deemed noble? For instance, in a mature feudal society, one that has been around for centuries, it could come to pass that one needs to be noble to inherit a holding, even if one is descended by blood to a noble. Just as an individual with a given surname may not necessarily inherit the right to a coat of arms, so too might a distant relative not legally be deemed to be a sufficient claimant to property belonging to a noble. For your campaign world (aka P-Harn), you can have it come to pass that a knight dies, and his second cousin inherits the holding simply because he's a member of the clan. However, if it is possible for a commoner to be knighted (and thus enobled) while his cousins, uncles, and even father - remain commoners. A baron for instance, might have a manor that he enfeofed to a knight, but would not honor an uncle taking over the manor by "clan rights" - simply because the uncle is not a noble born individual.
As for all other lands, the lord holds them for knights service and court. This means if he chooses not to enfeof that land to another knight that he still has to provide one if the feudal muster is called. There are some who have suggested that since the bailiff may be a noble that he provides the service. I hold that if a bailiff is required to do so and still pay a larger share of the proceeds per the rules how can he maintain himself as a mounted knight? In addition, what man would provide knights service for managing someone else's land with the prospect that he can be bounced whenever the lord desires a change in management. No, most of these extra knights are household knights in the baronial households and knighted landless clan members in the clans that hold such lands.
If you look at the information given about baliffs in published Harn material, it does indicate that the bulk of them are usually family members. When you consider that one can hire a knight for one's household by paying 3200d per year, and that one can have household members take over the role of a Yeoman, it would appear that the distinction between a Bailiff and someone who "owes" military service is very definite. If a noble owes military service for the land he holds, then he holds the land as his own. If he maintains it for the benefit of another - he is exempt from military service and obligations - and as you mention, be summarily removed at the will of the landholder.
As in parts of southern France, some regions did not partition their lands to support younger members but held the land in whole and supported those members within the household. Maybe some of our French members can shed some light on this.
I had always believed that the English version of feudalism, was a variant of the Continental version simply because of William the Conquorer's assumption of the English throne by conquest, and subsequent imposition of French nobility upon the anglo-saxon population to the extent that French was the court language of England in the early years, and continued on well past that start. While the variations are there to be sure, I hadn't really thought it would be all too different than the English feudalism. Mind you, the French had a lot more "allodial" issues than did England did - but that's a story of another kind