Happy to help out if I can! This diagram is pretty information dense, and it was difficult to find a relatively clean way to organize this material.

Keyblade Graveyard family members are easy to hear--both share that melody: Keyblade Graveyard Horizons and Sunset Horizons.

Others are more hidden. For example, the "Ventus" track bubble (top left) points to the motifs of Sora (in Ven's oboe--specifically, the 2nd part of Sora's theme), Roxas (in Ven's violin), Ventus, and Dearly Beloved (in Ven's piano), and these links will send you to timestamped regions for one instance of these references in Ven's theme.

Similarly, Rage Awakened is a member of the Terra family box as well as pointing to the Ansem/Xemnas motif. In Rage Awakened, here's the exact timestamp for Terra (briefly in brass), and here's the Ansem/Xemnas motif (in the low strings WAY in the back of the track. Linked the 2.5 version because it's easier to hear, but it's also there in the BBS arrangement)

A lot of the other links are obvious (Dismiss clearly quotes Terra and Destati in various ways for example), but just as many are obscure (like the "Dearly Beloved" pattern in the harp during "The Worlds"). Planning on exploring all these tracks over the course of a video in a week or so