Man. This is a muck of a situation, which isn't really anything new in this quest.
So my issue with the Storm of Iron strategy is that mixes poorly with the diplo options. Obviously getting the 2000 hoplites out of Thurii is key to its success. Beating Taras on the field and all that, but I don't think we can get them. Sending Mnemnon to Metapontion isn't a guarantee of success. It's just a chance at one, and I don't think he'll succeed.
We've got this well deserved half-barbaroi reputation, because voters just don't have the strong Hellenic supremacy beliefs that the NPCs do and have acted accordingly. Even goes back to the initial "free the slaves and enfranchise them" choice at the beginning, as our citizen base became a lot more multicultural and diverse than is expected in Ancient Greece. We're an oddity, and we continually affirm those suspicions with all our barbaroi befriending. And so they want to meet with the xenoparakletor, which is a lose-lose situation. If we say no and send him to the Dauni, we're just reinforcing this bad reputation. If we send him, well...
Cetashwayo mentioned last turn that negotiating a deal with the Metapontines is not Mnemnon's strong suit. We elected a xenoparakletor to deal with barbaroi, and he did a good job of it. Earned himself a permanent position as liason to the Peuketii were that an actual thing. But now we're asking him to go talk to some Greeks and he's no a model austere Hellene. Glory 2 means he probably won't make a great impression. The Storm of Iron strategy also makes his job a bit harder, because we're going for a more decisive victory which they are nervous about. We are going about it in a very Hellenistic fashion, but all the same, not giving them what they want. And a poor salesman at that.
There's also the matter of opportunity cost, where what happens when we don't make promises to the Dauni? Well, Storm of Iron takes a lot more of our manpower away to fend them off at first, but Serpent's Vice keeps more of them locked up in siege. And again, this assumes that the mission to the Dauni is successful and our xenoparakletor is not completely swindled and outwitted by the apparent intrigue-hero king of the Dauni. I do fear the message that sends, where we decline the Metapontine's request and instead grant that of a barbaroi. Which we then have to weigh against sending a poor representative.
So yeah, losing out on the right of passage is a blow to success chances on that one. A shame, because I think Storm of Iron's very workable with Thurii's support, but so be it.