"In European diplomatic history, Lannes's brief and unconventional ambassadorial career is either mentioned as an amusing incident or ignored altogether. The diplomatic community perceived him as a rude, upstart military man, hardly someone to take seriously. Lannes never had a family member to write a detailed and favorable account of his sojourn in Lisbon, so the best evaluation of his diplomatic efforts comes from his dispatches and correspondence in various archives. Writers of memoirs provide the local color but, as usual, their prejudices and the exigencies of their times influenced what they wrote."
I have only read the article (and not the relevant chapter) dealing with Lannes' two assignments in Lisbon. It presents an interesting if decidedly one-sided perspective, but not for the reason alluded to here, which is perhaps a reference to Laure Junot, duchesse D'Abrantčs - whose memoirs are cited a couple of times but was not present in Lisbon contemporaneously
There are but two or three references to contemporary Portuguese correspondence - for example AN AF IV, Dom Joăo to Bonaparte - this one hardly able to shed any light on discussions or occurrences in Lisbon
And the Portuguese histories referenced are very dated see for example fn 41. Angelo Pereira, Dom Joăo, Principe e Rei. 2 vols. (Lisbon, 1953), I, 101; Damiao Peres and Eluetério Cerdeira, História de Portugal. 8 vols. (Barcelos, 1928)
One of Lannes' achievements was to obtain the dismissal of Almeida and the appointment of Balsamaő (sic) - the former described as "inimical" (as are all Portuguese diplomats and courtiers in this article) and decidedly pro-British whilst the latter, if not pro French, more amenable to French views. Rarely do we see in history an ambassador to a court presenting such demands as the dismissal of secretaries of State - Almeida at this time held the equivalent of the position of prime minister at court.
Lannes presented Dom Joăo with a list of demands to which he expected a "yes" or "no" response - hardl;y diplomacy's finest hour. (Margaret) Chrisawn reports that even "Balsamaő wrote to the Portuguese ambassador to Great Britain that Lannes's "extraordinary note ...seemed more an anticipated declaration of war than an official note expressed in suitable terms." " fn 40. PRO/FO, Portugal, 63/42, Balsamaő (sic) to de Souza Coutinho, 11 September 1803.
What we have is a decidedly bellicose and bullying ambassador, who (ultimately) is able to impose his will despite Portuguese attempts to remain neutral (placating requests where possible - giving in to demands where necessary).
What would have been extremely useful would have been some insights from a Portuguese perspective to enable us to understand the shift in Portuguese policies - if not from contemporary documents at least from more modern histories dealing with the period.
The edition here is 2001 https://books.google.co.uk/books?id=k0NDkb1RNOIC - if this is the publication date, some insights from modern Portuguese history could certainly have been incorporated.