My take is this: Tui Manua was an off shoot of the Tongan Tui Tonga, most likely a younger brother who left to start up his own.
Once Tui manua was estabilished The Tui Tonga used this connection to infiltrate Western Samoa through intermarriages, a tried and true method of expansion for the Tongans. With this Tui Manua gained power over Atua and Aana, the real Samoan chiefdoms backed by the Tongans and their allies the Fijians. Even the ceremonies surrounding the Manuan chief are traditions that mirror the Tongans.
If you look at how and where Talakaifiki set up in Western Samoa according to Samoan history the Tongans started from the East, coming from Tutuila and made their way inland to Upolu and Savaii setting up forts along the way. That means they did not sail directly from Tonga or Fiji where it would be faster to reach Upolu and Savaii but went all the way around to Manua and sailed in from the East. Why? Was it because they had already estabilished a base in Manua? What other reason would there be?
When the Tongans were defeated, Atua and Aana, the true Samoan chiefdoms were restored while Manua was left out of the picture entirely. This is a glaring fact that should be looked at closely.
The logical explanation at this point is Tui Manua was at its core a Tongan chiefdom in Samoa set up through intermarriages and was later used to infilitrate the Samoan ruling families.