The Ferengi actually fluctuated later on as well, enough for us to declare their society a varied, multifaceted one. This isn't "early" weirdness regarding the species, it's just plain weirdness. And the aggressive, violently opportunistic Ferengi return in e.g. "Peak Performance" and "Acquisition"...
As for the "Ferengi custom" of selling out a comrade, it's probably a very real one - only, our heroes see it in slightly more concrete terms than usual here. Just like the idea of the Ferengi eating their trade partners alive at the negotiation tables is probably very much a true story, even if Riker's version of it in "Encounter at Farpoint" is somewhat misleading...
What Counselor Troi can or cannot read is always a source of amusement. But here, Deanna outright says that she cannot
"sense" the Ferengi. It's in "The Battle" where she seems to wrangle some sort of information out of the Ferengi opposition, "sensing" at DaiMon Bok "deception and danger".
Weirdness relating to Troi in this episode includes her considerable insight: "We have ignored the planet". This is a dramatic revelation, accompanied by dramatic music and a dramatic cut - but since it's coming from Troi, everybody ignores it as usual, and in the next scene the heroes are humiliated and surprised by the fact that the planet indeed is responsible for the situation.
But why did the heroes ignore Troi? It wasn't a Betazoid insight she offered - it was a verbalization of a conclusion that everybody in the room
should have already reached, as they witnessed their ship being savaged in ways that should have been beyond the Ferengi means.