Considering killing Maeva would be in the interests of Levateinn, I say we don't give her the satisfaction. That said, we shouldn't reject this offer outright. Pretend to go along until we can get back with the rest of our party. The guards sent with Yulia are dead, so make the case that Yulia would need her party to take on Maeva, we need something to give to the Sahaguin king. Ask for the weapons the shells would have gotten us, and say we'll trade some to the king to get the party back and use the rest to kill Maeva. Then, we seek her out, but reveal what's going on to her. If she knows the weapon seller is an ally of Levateinn, maybe she can help us deal with him and/or turn the three factions against him.