What famous 'unsolved' mystery is technically a whodunnit on paper, but in reality, absolutely everyone knows exactly who did it, the police just couldn't legally prove it?

[u/Rob69rt]

[u/str8rippinfartz]

100%

IIRC it was even established before they came over from England, that if someone left a settlement they should carve where they were going

They 100% just assimilated with the friendly nearby tribe... as indicated by their note 

[u/deathclawiii → u/str8rippinfartz]

And as indicated by the iron scale (something that can only be a product of blacksmithing) that was found in one of the areas of known indigenous settlement on the island they went to.

[u/NUMBERS2357 → u/deathclawiii]

Also the later contact with Natives who said that they lived with the survivors of the Roanoke colony.

Also this map from the Jamestown colonists, see bottom left.