Find a journeyman blacksmith

Complete the quest: "Ladies of the Wood".

I finally found the Baron's wife, it turns out the part about investigating all remaining leads refers to the leads about Ciri. There are two quest series about Ciri in Velen, and at this point you have to follow the other quest series to complete the "Family Matters" quest, and "Ladies of the Wood".

I was searching for the same but having explored what I believe was the whole area of Kaer Morhen, I could not find them anywhere either. My conclusion is that the chronological implications in this game are simply badly implemented because this was not the only occasion when the plot didn't make sense and expected characters to be somewhere where they weren't, for example

Triss in Novigrad during the Redania's Most Wanted quest or Yen in Skellige during some of the side quests there.

I believe that sending people to Kaer Morhen serves only to prepare for

the Battle of Kaer Morhen (the Brothers in Arms: Area quests)

and the game doesn't acknowledge their presence before the respective main quest.

The story in this game is very enjoyable but at the same time it's very easy to miss important scenes. For example, I completely missed

Letho

and never expected that something as simple as

bringing Ciri to see Emhyr var Emreis

would result in a completely different main ending.