I doubt I can add anything new to this discussion. But while I know it is the crazier option, ever since finishing OB I've been suspecting something more like Option 3.
I don't feel that he is becoming a new Adonalsium, but I have a personal theory that he is being influenced by some sort of remnant of it for a greater purpose.
The references by Odium 'we killed you' his talking about being Unity, merging 3 realms etc. He hears the voice saying 'Unite them', which forgive me I don't have the evidence this second, but I'm sure Dalinar says he doesn't recognise it. He has heard Tanavast speak in visions, so if it were part of Honour I think he'd recognise.
The other big thing for me was at the end, saying to Navani that he can see a 'second light' which is far away.
I think that some part of Adonalsium still remains in the spiritual realm and is influencing Dalinar. This is the voice and the light he hears/sees. Possibly this power is still manifesting through shared investiture from the 3 shards, as other ideas have stated, but I think the major influence / drive is coming from this remnant of Adonalsium.
We know very little about the Spiritual Realm, and Adonalsium. But given how powerful it would have been pre-shattering, and the inherent knowledge it would have had, I wonder if some part of it would have managed to remain. That it was so strong that it could never be truly destroyed, like it's power could only be shattered, and its greater being or mind remains in the spiritual realm.
All speculation, I know, but that's where my mind has been going since the end of OB.