New crazy theory time!
Given that I find Jacen post-TUF to have been handled in an immensely unsatisfying way (JINO), it occurred to me that Luceno left something on the table in the novel Millennium Falcon that either is a vestige of a discarded plot element to FOTJ, or hinting toward this theory, or some variation thereof. Namely, in MF, when Seff does his thing at the zoo, Allana identifies him as feeling like Jacen in the Force. At the time, it was believed that Jacen was somehow related to the insane Jedi, given their demonstration of abilities he knew, e.g. Flow-Walking (or more accurately, Aing-Tii fighting-sight?), among other things.
However, it has subsequently been revealed that Abeloth is the cause of this, at least to some degree, given its cessation upon her apparent demise -- which led me to erroneously believe her to be well and truly dead, given that Luke had no way of knowing the psychosis ended thus making it a feint seem unlikely. But I digress.
Anyway, the fact that Allana identified an insane Jedi as feeling similar to her father in the Force would seem to be foreshadowing by Luceno for something which was later dropped, or something still in play. My previously stated belief, that Abeloth is incorporeal and takes possession of individuals, would therefore provide the possibility that Jacen befell this fate, and that JINO was in fact Abeloth in Jacen's guise.
Of course, this theory has holes, namely JINO didn't behave in a manner that would benefit Abeloth based upon her current behavior and she's trying to undo what he did, and his ghost was still kind of evil, so...
Also, don't know if this has been mentioned, but
Click here to see the hidden message (It might contain spoilers)
Since Abeloth has the stated goal of locating the Jedi Queen, and Allana has revealed this information to the Barabels in front of Bazel Warv, whom was afflicted with the psychosis, I think it's very much likely that Abeloth now knows the identity of the Jedi Queen.
Edit: It has been pointed out to me by an esteemed colleague that the ghost of Jacen could have been an illusion generated by Abeloth, given the uncertainty around how genuine those apparitions truly were. Further, Abeloth's stated goal of restoring the flow undone by Jacen could have been a lie to placate the Fallanassi. Don't stop believin'!
"Trust not the words of a poet, as he is born to seduce. Yet for poetry to seize the heart, it must ring with the chimes of truth.”
“The world is understood through metaphors. Language is a metaphor-system. Mathematics is a metaphor-system. All real-world schools of magic and religion revolve around the understanding of vast metaphor-systems, symbols as they relate to concepts."
"See, the thing is, everything everyone tells you is a lie. The truth is always bigger than the words we use to describe it."