Blackbeard already knows everything about his lineage imo, he can use two devil fruits thanks to his knowledge of his particular lineage, I think the mission in Egghead is to use Catarina devil fruit with Saturn ( with Saturn appearence you can use the Seraphim, a more great power for Blackbeard)
Blackbeard is the man with most power at this point if Caribou joins him ( ancient weapons)
maybe not everything,he was an orphan so maybe noone was there to tell him the whole story.
On top Teach is interested in History as his hobby,and we know OP history is missing crucial information.
Seraphims would be an huge Edge in BB Arsenal,on top with Caribous knowledge as well.
Just read a theory on FB today about the part why Devon touched Saturn.
It‘s about the quote Shiriyu said while they met Kuzan about the man with the burned scars.
"Maybe this man is affiliated with the WG "and they need Saturns appearance to get in contact with this man .