We are on thin exegetical grounds when we base too much of an explanation on a linguistic feature in the text, but there is the possibility in this case that the presence of “these things” in the plural is actually significant. Alma is letting Corianton know that he has done something that is an abomination in the sight of the Lord, but it does not appear to be a single thing, but rather “things” – in the plural. What could be meant?
When Alma gets more specific in verse 11 he again mentions that Corianton should not consort with harlots. That is clearly one “thing.” However, Alma also notes in verse 11 that he should not be “led away by any vain or foolish thing.” It is probable that Corianton’s abominable sin consisted of two parts, begin led away by a “vain or foolish thing,” and also consorting with a harlot. When we place these two “things” in the context of the religious/political worldview of the Lamanites, and the possibility that the named harlot would be a priestess representative of the new religion to which Corianton had apostatized, then we see how the confluence of these things become the abomination before God.
This is not a simple error of sexual passion. That would be a sin, but certainly a forgivable one. It hardly fits the category of “most abominable above all sins save it be the shedding of innocent blood or denying the Holy Ghost.” What would be “most abominable” however, is the denial of the gospel and the embracing of a false gospel. Such a sin would preclude forgiveness not because it was not possible (as with the sin against the Holy Ghost, or the shedding of blood) but because the person who had apostatized would have put themselves in a position where they had lost their belief in the repentance process, and would not repent. One who will not repent is only slightly less in condemnation than one who cannot.