Would there be legal wills in D&D?
For anyone with little or no wealth, the answer is a clear no. Not that there wasn't property, or that poor people have none of it, but because in general property among the poor is a family matter, and not something dictated over by the state. A will is a legal document, and a Medieval family would not have needed the state to determine the outcome of something that was considered a family decision.
This would have been true for many people with wealth, as well, particularly when that wealth was in the form of land. The pater familias tended to have authority both in eastern and western cultures, no matter what the father may have wished for upon the father's deathbed. It is rare that the second son succeeded to the father's wealth if the first son was healthy and capable.
However, with the increase in international holdings, in both the New World and in Asia, and the complexities of arrangements with companies as opposed to guilds and families, the matter of a wealth distribution after death became more complex ... but I would argue that while documents are necessary in our world, the world of magic would have a natural bypass.
A substantial portion of the legal profession would employ clerics who were able to speak with the dead ... who could substantiate documents with the words of the author, spoken directly to the family if necessary, in a much more interactive environment that mere video recording. Family members could ask, specifically, about hidden hordes and property that slipped through the cracks of notation ... and get the word directly from the source as to who should get what. Such events (called "audits," obviously) would serve to fix beyond question the matter of ownership, even in cases where the dead might have to be spoken to for months before ALL the matters were finally settled. Lower level clerics could deal with minor personages; and high level, 12th+ level clerics with difficult matters of nobility, overseas companies and so on. All such audits would have the power of law, and would almost certainly not be required to present in court. After all, the final judgement would always be made by having the official witness, the dead, make the final statement.
Where the dead were unwilling to be forthcoming, the matter could be resolved by having standard policies of distribution, or perhaps calling on the gods for a divination about such. There could also be a possible appeal to the ancestors of the dead, if it was noted that a person had been in possession of some property for a short period of time, perhaps a six month minimum or such. Laws could define these things. If a horde was forgotten by the dead, the family could inquire about any object in the horde and find it with locate object, or by using augury if necessary to narrow down some of the details.
Much less wealth would disappear from the economic well-being of the land, and there would be a clearer established method of inheritance ... which people would adapt to with all the certainty we have about more rigorous, concrete matters of law. There might still be documents, but when a D&D Courthouse speaks of "probating," they don't mean doing it in the dark as we do it.
This matter of speaking to the dead has many ramifications. For example, imagine the climate after a large battle has taken place. Just as in our culture, lists would be made of the dead, of the ranks of the dead, and of the missing in a battle. Armies of clerics would set upon the enormous task of speaking to all the dead, finding out how they were killed for the record books, who killed them, and of course the above matters of inheritance. Some cases could drag out for months. As well, every person on the missing list would be contacted through the spirit world - and if they were not actually dead, the clerics responsible for this activity would KNOW it.
A list of names of missing and certainly alive would be massed, to be handed to another bureau whose responsibility was to find out where they were, if they were being held prisoner, if they were even on this plane of existence ... and carefully tracked to see if they had died this past month or not. Slowly, steadily, all such missing persons would researched, in part by magical means, but also by agents who had certain charms and talents than enabled them to learn more about the missing. Of course, some minor persons, without names, could go missing after a battle and no one would care ... but anyone with a name, anyone who was an officer or official, or attached civilians with income, would certainly be searched for and - this is where D&D departs greatly from our experience - absolutely and without question FOUND.
There might be a point where lost persons were obviously moving away, seeking not to be found, and some risk/value added assessment would have to be made ... but the only persons who could not be found in a world of spells like those available would be those who had other spells they were deliberately employing so as to disappear.
Thus, no family would need not know what happened to their son after a war, if they cared to enquire. There would be services they could turn to, and eventually stand next to the mass grave where their boy was interred, or know for a fact that Jack or Benny or Don were in South America, making their fortune.
Such is the power of magic.