Trying to sort out what he means by "Innocence" and "Experience" in the context of the songs titled with those terms is much harder than the superficial simplcity of the poems in these collections would suggest, though the weirdness I've outlined above gives some reason to expect that such might be the case. The two Introduction poems do seem to hint at a resolution. Here are the poems and then a few (admittedly disjointed) thoughts about what we can draw from the two.
Introduction to "Songs of Innocence"
Piping down the valleys wild,
Piping songs of pleasant glee,
On a cloud I saw a child,
And he laughing said to me:
"Pipe a song about a Lamb!"
So I piped with merry cheer.
"Piper, pipe that song again;"
So I piped: he wept to hear.
"Drop thy pipe, thy happy pipe;
Sing thy songs of happy cheer!"
So I sung the same again,
While he wept with joy to hear.
"Piper, sit thee down and write
In a book, that all may read."
So he vanished from my sight,
And I plucked a hollow reed,
And I made a rural pen,
And I stained the water clear,
And I wrote my happy songs
Every child may joy to hear.
Introduction to "Songs of Experience"
Hear the voice of the Bard,
Who present, past, and future, sees;
Whose ears have heard
The Holy Word
That walk'd among the ancient trees;
Calling the lapsèd soul,
And weeping in the evening dew;
That might control
The starry pole,
And fallen, fallen light renew!
'O Earth, O Earth, return!
Arise from out the dewy grass!
Night is worn,
And the morn
Rises from the slumbrous mass.
'Turn away no more;
Why wilt thou turn away?
The starry floor,
The watery shore,
Is given thee till the break of day.'
So, if Imagination is a concept through which Blake is attempting to effect a reintegration of the human faculties of thinking and feeling (and Imagination seems to be the actualization of the poetic genius, which is the capacity of realizing man's spiritual potential), could the divide we encounter between the two books be something of Blake's recognition of the persistence of a certain tension between thinking and feeling even when they are both given their proper due as integral powers of human Imagination? Not that feeling belongs exclusively to Innocence or thinking to Experience, but each state of the soul may, it seems, be in some sense seen as leaning towards one aspect of the human capacity for Imagination. In the Songs of Innocence, that might be one reason for the heightened musicality, clearly-seen "narrative" (that term used very loosely), the very pictoral encounters with nature, the explicit description of the poetic voice as that of the "Piper". And in the Songs of Experience, the emphasis on thinking may be seen in the omnipresence of human buildings/attempts to order the world, the increased willingness to think deeply about the ramifications of actions, the awareness that initial impressions of goodness and beauty may be superficial, and the identification of the poetic voice as that of the Bard (whose words are more important than the music with which he accompanies them).
In all this also, there is a possibility of understanding Blake's comment in "The Marriage of Heaven and Hell" about the necessity of contraries for progression. Each state of the soul cannot be present in the same person at the same time, but neither one can be accepted as the only positive state of the soul: you need to accept that both get at a part of the truth and that the tension between the two is necessary for any progress towards a greater spiritual understanding.