What I have difficulty understanding is whose disadvantage and in what regard? At first I thought the reference is to Mario's disadvantage in his competition with the other youth for Silvestra.
On the other hand, maybe #5 & 6 are right, that Mario's disadvantage was that of being in a trance.
What was heard in that awful span of time, in which all relations between happiness and illusion crowded in upon one's emotion—not right at the beginning of it but immediately after the sad and ludicrous union of Mario’s lips with that loathsome flesh that had fobbed itself off on his affection—was the giovanotto’s laugh, on our left, the only sound to pierce the tension, brutal, gloating, and yet, unless I am very mistaken, not without an undertone, a hint of compassion for Mario's having been put at such a disadvantage while in a trance, and was not entirely without an echo of that cry “poveretto!” that the conjuror had earlier condemned as misdirected and had claimed for himself.
It might be shortened to "such a trance-induced disadvantage" or the like, as Bion suggests.