The only constant right now is that Argurios should sacrifice to Hephaestus, probably Wealth due to the character of the god. Somewhere between 2-4 to show that we really need this, but not too much or every Olympian will start milking us.
Isidoros can build or organise the court. If we look at buildings, there are 4 open slots for now (13/17). But we want a quarry, later a palisade, probably a winery and a temple to at least one of the major gods. However, the temple is locked behind the quarry, the quarry is unsafe until the thorny beast is defeated, the winery cuts into our food income too much (if we convert, our supply income avg. will fall to 16,5 < 17 consumption, but we can trade for supplies?), the palisade does not seem to be an urgent need.
So winery or court. Probably court, especially if we promise Argurios/Semni lordships or the king's position.
Semni is not a good candidate for building (see previous revolt), but what else there is to do? Perhaps local legends - try to find out what the forest beast is.
What do Isidoros do to help Semni and Argurios find peace?
I am wary of picking anything except for 'do nothing' here because Isidoros's bad diplo may mess things up again.
Edited vote:
[X] Grant them wide lordships over the lands of what would one day become the Hyphyrian hinterlands. Though the lands are wild yet, the prestige the announcement will bring will do much to quiet whispers about Semni's rough origins, and it will please Argurios' pride
-[X] The sparse forest to the north of Hyphyria on the banks of the river.
[X] Isidoros
-[X] Organize a proper court.
[X] Argurios
-[X] Sacrifice to the Gods [Locked in for Argurios]
--[X] Hephaestus
--[X] 5 Wealth
[X] Semni
-[X] Study Local Legends