Humans would have to do it anyway in the normal run of things and to a lesser scale. Humans breed quicker and are not a dying race spectacularly ill equipped to fight in forests.
I mean, option's description is "find alternative routes and pay money to Empire", so I am actually not quite sure where the whole "mass casualties" thing came from.
Really, what's up with the idea that economic response is going to be bloodier than naval war? Did I miss some WoGM that the "economic help" option is somehow "Karaz Ankor takes and holds all land and mans every trade post no matter cost in lives"?
This does not say mass casualties though? This says "costs influence to reroute trade so that it costs less to Empire to live in Marienburg blockade". Securing routes involves some risk, likely, but this is manifestly not a citation on mass casualties.
If the option involved something like "KaK marches on Sylvania and Badlands and loses thousands to secure trade", or something of similar magnitude, I am rather certain it would be mentioned explicitly, and not as "influence".
Edit: really, idea of economic help being more bloody than, you know, war, is just weird. Wars are not known for being peaceful, even limited ones.
They also have a bad habit of not staying limited, especially once weird alliances are invoked, but that's another matter.