3 Nephi 16
I just realized that the Gentiles, generally speaking, have/had accepted Christ – even if their knowledge of him was incomplete. When Christ talks about the Gentiles rejecting the fullness of the gospel he is talking about a scenario when modern Christendom as a whole rejects the doctrine of Christ – the pattern is likely to be similar to the way the Nephites, as a whole, rejected the gospel after having received it in purity from the Savior.
My suspicion is that the some leaders of the various Christian sects will convert Christ from being the Son of God to being a great philosopher or a valuable myth. Among the followers of all the sects (Latter-day Saints possibly excepted) the majority will remove Christ from their daily lives and become idolaters or functional atheists. Among the Latter-day Saint the same will happen among a large portion of the saints. It may be more than half, but hopefully it will be well under half (I would be surprised if it was less than 40%).
The prophecy that the watchmen shall see eye to eye could refer to union or agreement between the watchmen of various covenant groups, the Latter-day Saint leaders representing the watchmen among those remaining faithful of the Gentiles and the watchmen among the Jews and any other covenant groups (possibly separate watchmen among the ten tribes from the watchmen among the Jews).