Isn't the mirror stage really about discovering the unity of the body (as a kind of closed contour), thus permitting entry into the Imaginary? Then the Symbolic is initiated when these entities (of which the body's image is supposedly the first) start to be thought of in relation to each other, as in language. In that sense, I don't think you can have language without the Symbolic, so the analogy with Jaynes fails. Jaynes didn't claim that the bicameral phase meant undifferentiated reality without terms or objects.
As I understand it, Lacan is talking about individual psychology. It doesn't make sense to talk about a person whose mind is "fused" with social reality, unless you believe that reality is some kind of solipsistic hallucination.
About entering the Imaginary: you're in the right.
That said, there are a number of stages in the development of Lacan's thought (Malcolm Bowie counts three); he starts out ostensibly as a clinician but gradually expands his concepts outwards until they're a generalized theory of subjectivity.
From this point we can argue whether societies have structures somehow resembling subjectivity - as an economist I've spent a lot of time on the clock doing this. The thing is that as a clinical theory Lacan is way out there in the sophistic hallucination scale, even for the tiny minority that wants to resurrect psychodynamic therapy. Yet that's not what draws people to him, is it?
Well, I've never heard of Lacanian economics, but there are plenty of fields in which understanding a bit about Lacan is essential if you want to appreciate the canon.
Lacan as a person/analyst doesn't seem appealing to me, but some kind of psychodynamic therapy surely has a future in the long term, as insight accumulates into the mind. So Lacan seems like something to cautiously sift through for insight.
The social as a subjective entity, or at least an apparently somewhat integrated/regulated entity that individual humans can interact with (the big Other), certainly exists as a concept in people's minds. It's a very formalistic, broad-brush picture of social interactions, of a kind that would probably appeal to computer scientists. There's not a lot of scope for nuance at the level of individual interactions. From that point of view, I suppose it could resemble the crude regulative ideas that control the bicameral mind.
As I understand it, Lacan is talking about individual psychology. It doesn't make sense to talk about a person whose mind is "fused" with social reality, unless you believe that reality is some kind of solipsistic hallucination.