[] Let the Highlanders extract their vengeance and attempt to make amends after (-1 Legitimacy, -3 Diplomacy, possibility of war with Highlanders, chance of spawning new faction in the lowlands)
Legitimacy can be solved with a main proclaim glory (which will solve our stability to a degree), the diplomacy is a small cost, the possibility of war gives us a way to vent our martial anyways, and the faction of warriors that have iron don't know how to make said iron...and they'll have those weapons break eventually.
[] Attempt to capture the errant warriors and then make amends with the Highlanders (-5 Diplomacy, probable war with Highlanders)
Highest diplo cost with highest war possibility (besides it being a sure thing). This will likely allow us to find the guy who organized it and arrest/kill him. Arresting or killing him will likely cost stability/legitimacy as he/she's likely a war chief or someone in an influential enough position to get all those warriors over there.
[] Reinforce position (Conquer new province, war with Highlanders)
Starts the war personally, most likely drops our centralization because of the new providence, opens up cultural friction from the conquered people. Makes us vulnerable to a stability drop if we lose that providence from the HK. WIl likely demand a secondary war mission to keep the province.
[] Massively reinforce position (-5 Military, founds new March, war with Highlanders)
Creates a new martial colony to vent our martial and will go to war for us while we do our own thing. Will require 6 diplo to integrate it later, support because the HK have walls, will remove the buffer between us and the lowlands crap fest.