ὥστε τί ἂν λέγοντες εἰκὸς ἢ αὐτοὶ ἀποκνοῖμεν ἢ πρὸς τοὺς ἐκεῖ ξυμμάχους σκηπτόμενοι μὴ βοηθοῖμεν;
I think there are two parallel clauses which share τί εἰκὸς:
Saying what reasonable [argument] would we hold back?
Relying on what reasonable [argument] in the face of our allies there would we fail to come to [their] aid?
τί εἰκὸς is the direct object of both λέγοντες and σκηπτόμενοι.
But I think you could go either way on whether εἰκὸς belongs with both participles. But τί, in my mind, does belong to both.
What is the relation between ἀποκνοῖμεν and ti?
I don't think τί εἰκὸς has, or needs to have, a relationship to the main verbs ἀποκνοῖμεν and μὴ βοηθοῖμεν. It's the object of the participles, not the main verbs.
Does the word order make (ii) (a) more likely, in that the fact that εἰκὸς follows λέγοντες perhaps makes it seem more like part of the first clause?
I think τί εἰκὸς is best taken as a unit, notwithstanding the word order, but perhaps you're right. It doesn't make a big difference in meaning, But I think the idea of a reasonable argument is just as inherent in the second clause as in the first. The idea of the second clause is "how can we rely on any reasonable argument in response to our allies there and fail to come to their aid?" σκηπτόμενοι seems to add a suggestion that any such argument would be disingenuous.
If any word is out of its "natural" order, it's λέγοντες, not ἢ. The structure would be clearer to us Anglophones, perhaps, if the sentence were re-written as:
ὥστε τί ἂν εἰκὸς ἢ λέγοντες αὐτοὶ ἀποκνοῖμεν ἢ πρὸς τοὺς ἐκεῖ ξυμμάχους σκηπτόμενοι μὴ βοηθοῖμεν;
I think it's important to keep in mind that Greek word order (like Greek modes of expression generally) is different from English word order, and that we should be careful not to impose on the Greek what we may think of as a "natural" or "logical" word order being native speakers of English and definitely not of ancient Greek. Some of the books tend to speak in terms of departures from normal or logical word order, but we should resist. That's why "trajection," which implies that ἢ "logically" belongs earlier in the sentence, is a somewhat misleading term. (And the position of ἢ after εἰκὸς is another clue that τί εἰκὸς, and not just τί, belongs with σκηπτόμενοι and not just λέγοντες.)
at 6.18 there is no article and no dative.
shows up further along in the chapter, in 6.18.4, maybe something like this: "and, at the same time, we will in all likelihood
rule all of Greece, once those there have been added [to our territories?], or we will harm the Syracusans, in which we and our allies will benefit."