Thank you for your long and thoughtful post.
Let me quickly point out that my final sentence was not to in any manner justify any side's actions, let alone the French. My posting was exploring what Marten wrote as his writings (they are easier than those of Vittel).
The final sentence was meant to communicate (to my astonishment) that barbarous acts are forseen within Marten as allowed - if that is the right word - under what he called the "laws of war" (Vittel as well from my reading).
The balance of any discussion relates to two (or more) different views of "their side" of the situation ("legal" situation if one wants). These differing views are in conflict and there shall be no reconciliation of them. At best we can "understand" each side's view.
With this in miny to turn to your first point (the passage quoted is is from Martens not me), the information you provide is more than I have gathered. It makes sense from the Anglo-Portuguese perspective. It suggests the intention was defensive.
What remains open are the actual actions and the French view of those actions.
I do not have specific material for the French side.
I do not have sufficient material on actual actions by the Portuguese. (I would just use the reverse of Reuters view of "one man's terrorist is another man's freedome fighter" to say any "heroic act" by Portuguese - whether ordenança or peasants - could provide the French with justification - I write could provide justification and not they would justify - the barbarous acts as "foreseen" in Marten. However, "valid" the Anglo-Portuguese position, it only takes one act for the French to justify their own view.)
In your second point, I believe Wellington simply decided that the fastest approach to an acceptable solution (having spent time on other approaches).
Other recent posts on the forum suggest to me, that one side did not engage in acts otherwise "foreseen" in Martens (I keep using '"forseen" in Marten' as opposed to '"permitted" by Marten' since Marten is no authority, just a commentator) as, apparently, they forsaw the consequences of retaliation (as noted by Marten).
In Portugal and Spain, the "cycle of violence" and the meagre outlook for the peasants as well as the distance from home for troops of all armies created a volatile cocktail keeping retalion part of this war.
The same is true of parisan debates; once started, they continue.
Understanding is not justification; my goal is to understand these actions within their time.
Again, thanks for your post. It adds to understanding. - R