It is not possible for Betty to be anyone we know from the land of Ooo because in the episode 'Betty', Betty abandons her timeline to live in Ooo with Simon (which is why Simon never saw her after he went crazy). This would therefore mean that she wouldn't be there for any of your theories to turn her into people like PB, otherwise PB would have vanished when Betty left her timeline. However, the scientific parasite/bubblegum-like substance we see give the soup to Simon, is seen in the pockets of many scientists including Simon and Betty. Perhaps it grows into princess bubblegum (in the same way that PB can age rapidly from 13 to 18). This would explain PB's knowledge, likeness to Betty (it copied her appearance), and it would also explain why it smiled when encountering Simon.