@Vinylrider your understanding is correct, delta retention is just how many deltas to keep before merging the oldest delta with the full. Full backup interval is the time between each new full back up for use in case of corrupted back up.

To answer your question, yes it is recommended to have more than 1 full back up to fall back to in the event of corruption espcially of delta backups since delta backups rely on previous deltas meaning if one of your delta is corrupted, you'll loose all data from that point onward until another full backup is made, or worse your full backup could be corrupted (more likely because of the file size) then you loose the whole chain. And if your corrupted delta is merged with the full, you'll probably have a corrupted chain aswell.