Learn English – ‘He decided’ vs. ‘He had decided’

In general I agree that using ago implies that the simple past would work better. In this case, however, the simple past doesn't provide the obvious and relevant connection/link between the two sentences that had planned offers.

If it were me, I would use had planned (for its link to the previous sentence) but replace ago with prior to avoid any subtle semantic imperfections. (Edit: Except you stated that the semantic intent was half a year before "now"... So this suggestion is less useful.)

With that said, neither example is particularly offensive, and I doubt that other native English speakers would have strong opinions either way if they encountered either example in real life.

In the sentence you present:

"Since my forefather had said his name was Kin-tay (properly spelled Kinte), and since the Kinte clan was known in Gambia, the group of Gambians would see what they could do to help me."

The use of "had said" indicates that the forefather's action was already in the past at the time of the events then being described by the narrator. If you wrote instead "Since my forefather said his name was Kin-tay [...]" then the natural out-of-context interpretation would be that the forefather was present and gave his name in that moment, thereby directly influencing the Gambians to help. That the reader will recognize that interpretation to be impossible doesn't make it ok to substitute "said" for "had said" here; although the reader might understand the author's intent, the conflict between context and natural interpretation would be dissonant.

Are these two tenses sometimes interchangeable? Or is it a matter of duration? Have I missed something?

No, they are not interchangeable. It is a matter of the relative timing of events. When describing (simple) past events, one uses the past perfect to place actions in a more distant past time frame.