Profecturus is here the future participle of prōficio (to advance, gain ground, etc.) rather than of prŏficīscor. The two can be distinguished by the length of the o in the pro- prefix.
The cum clause cannot be concessive here. It seems to have both a temporal and a causal sense to it, but I think it is best translated simply as "when". It's also often better to translate parum as "not enough": "When he understood/perceived that he would not advance far enough/make enough progress against the Romans."
In Siciliam dicionis suae facere statuit, the genitive dicionis suae is the predicate of the verb facere, meaning literally: "he decided to make Sicily of his own dominion." It could be rendered more idiomatically in English as "to put Sicily under his thumb," or something along those lines.
Veho "to convey, carry" and its compounds are frequently used to describe the means by which one travels, here indicated by the ablative classe "by fleet". Praeterveheretur is passive with middle force to it, so literally "when he was being conveyed by fleet past Locri," but perhaps better rendered "when he sailed his fleet past Lucri." Lucri is one of those plural place names which technically refers to the tribe that inhabits the region.
Ex mala malo
bono malo uesci
quam ex bona malo
malo malo malo.