I have done a lot of thinking about lychee seedling variability (including the juvinile period). As people continue to select for desireable traits, those traits become more concentraited in the offspring. Thus a seedling grown from an ancient cultivar like Brewster will be much less likely to produce offspring with traits that people desire. Conversely, a seedling of a newer cultivar like Emperor/Chakrapad has had many generations to weed out undesireable traits and will be far more likely to produce the way people want it to.
Add to this that Brewster was thought by Groff to be a hybrid of Shan Chi (Mountain Lychee) and a "water lychee." This makes it quite variable from seed and is supported (at least to me) by the fact that one of the two trees we grew from Brewster seed produces fruit that looks exactly like the pictures (on USDA germplasm request website) I have seen of Shan Chi. Ie it is small, seedy, spiney, sour and took about 25 years to come into bearing.
The Emperor seedling on the other hand is exactly like emperor, leaves, tree, and fruit. It came into bearing quickly from seed and started blooming at only 6'-7' tall.
This would explain the disconnect between "expert" opinions. People came to different conclusions because they started with a different gene pool to begin with.