The conservation charity, which has invested £10 million in the Inner Hebridean island over the past decade, has been "bruised" by local community acrimony, the internal review says.
Four families have settled on Canna only to uproot themselves, pointing the finger publicly at the NTS's management.
The population had dropped to 11 but with the arrival of two families has now gone back up to 18.
The document says that for a stable community to be sustained, there should be no fewer than 30 people with a normal age spread.
It adds the NTS can claim success in the recovery of the seabird population and the restoration of Canna House's garden.
However, it is set to keep out of future attempts to attract new residents with the report recommending a new forum between the NTS and the islanders on the Canna Community Development Trust (CCDT). It says: "This will specifically exclude the Trust from 'engineering' a community development strategy given previous events.
"The residents themselves will take primary responsibility for their own future development and any expansion of the community."
The report says the NTS "has stuttered through several failed attempts to grow the population and achieve a stable social mix of residents, and been forced to deal with the resulting fall-out."
It finds there was a "natural tension" because the Trust was both landlord and employer "and the sometimes hard decisions the Trust had to make as the latter".
But it also identifies one of the causes of dissent and bitterness had been perceived "favouritism for the 'native' residents, whereas incomers have received short-term contracts of employment and/or limited tenures or leases".
It describes the relationship with community as the "Achilles heel".
Dr John Lorne Campbell had bought Canna in 1938 and he and his wife, Margaret Fay Shaw, moved into Canna House and remained until their deaths. They built up a unique collection of papers, audio recordings and publications which preserved what was then a disappearing Gaelic language and tradition. The island was donated to the NTS in 1981.
The report recommends the NTS consider the creation of new crofts, returning decrofted sites to crofting, or creating smallholdings, "with the purpose of strengthening the island community and supporting incoming families".
The vacant post of the resident Property Manager should be discontinued. There should also be review the role of the Canna House archivist, as the present incumbent is due to retire.
The NTS has resolved to develop a comprehensive conservation plan for the house and its unique collection of Gaelic archives.
Geraldine MacKinnon, whose family have been on Canna since the 18th century, chairs the CCDT. She said: "It is quite a good document. I don't think there is anything that we can't work round and there is enough in it to provide a basis for working on future development."
Patrick Duffy, the NTS Director of Property and Visitor Services said the review has helped it learn lessons and set out "the potential for a future course based on co-operation".