While Harry will never full become a warden, the sword is a powerful and capable foci, with its ability to attack magic. With the inevitable large scale combat approaching, such a force multiplier would be invaluable.

The stone table likely Unites the different courts and factions of the lands of faerie. Part of me wonders if since the fomor are the result of so many different groups, they have something akin to the stone table for themselves. To bind themselves together and pool their powers and energies.
 Since it was said the fomor were part faerie, I kinda expected them to end up being sacrificed on the stone table. So if the fomor did hove something uniting their power, could tHat be used to steal it for the stone table?

When Harry gets his sword, I see tons of highlander quotes and references.

The fomor were made into outcasts when their original homes were torn down, so it makes sense if they searched for a new patron, someone to back their play.

The descriptions of the well, make it sound like it is capable of containing the goin collection. Especially with a warden with soul hire, and a pseudo nephlim spirit of intellect daughter.

It is said Maggie was an expert of grey magics, so I wonder if she learned it from her dad. With the Blackstaff, it would be easier to experiment with safely.

In the arryns stories, there were three weapons. A sword, dagger and spear. If the sword of the cross is Excalibur, I wonder if the athame is the dagger.

McCoy might not be an active warden any more, but I could see him ensuring all his magical weapons and tools are well maintained. He would have the resources and talent to ensure his objects, are more long termed than what Harry has to depend on.