Thanks for mentioning me (although it embarrassed me).
On the “history of ideas” question — my understanding is that the first literary men to be interested in Indian thought would be the New England Transcendentalists.  (And then after that it enters public consciousness in the West, in garbled form, via the theosophists.  I looked up the Bhagavad Gita on Wikipedia (not the most reliable of sources, I know) and, indeed, Emerson’s name crops up, as also a relatively late date for the first translation (1785).  I expect the Jesuits knew something of Hinduism, because they seem to have regarded exact knowledge of existing beliefs as a vital prerequisite for conversion , and there was the Portuguese connection with Goa.  I don’t know for sure, but i doubt that anyone much else in Europe knew (or cared) anything much about Hinduism till a later period.  I think if it had been a topic of discussion, there’d be something in one of the omnivorously curious essayists such as Lamb or Hazlitt.  I don’t know of anything like that.
I think Nathan must be right to think that the source of the doctrines would be Neoplatonism — as rediscovered and re-imagined by Renaissance writers and filtered through English literature.  I’d guess the obvious source would be the Garden of Adonis in the Faerie Queene.  I take it that, in Spenser, this is a poetic device and he likely no more believes in it as “doctrine” than he believes that the River Medway could literally marry the Thames.  Wordsworth, however, *seems* to be taking the idea seriously.
But I wonder how far anyone would sensibly want to pursue that.  It seems to me that only people whose interests are fundamentally non-literary would look at a poem and try to extract doctrines from it.  If that were what mattered, then a list of statements in propositional form would “do just as well” — which is obviously ludicrous.  In any case, it seems to me that the movement of the poem is, as you said, thus: we start with an attempt to talk about a certain kind of experience and end with another experience, which seems different, although also intense.  Along the way, we’re offered a myth (whether believed in or not) that is, in a way, makes sense of the loss of the original experience.      The experiences seem to be what’s (potentially) most interesting.
Could the Shakesperian use of sleep as a metaphor for death be coming from the Tempest?
On the language — I understand the point that was made about Wordsworth’s role in simplifying poetic diction.  Wouldn’t it also be fair to say that Milton is a special case (as is Dryden)?  I think it’s interesting to compare the poem with this one of Traherne’s:
http://www.poetryfoundation.org/poem/180796
The diction there is simple; there’s also an interest in nature, although this comes from a much earlier period and a time before the intellectual historians would say people had “become interested in nature”.  (Aside: am I right to think a phrase in Kubla Khan might be coming from there? I don’t know if that was a poem published in Traherne’s lifetime and therefore available to have been read by other poets.)
Have you any thoughts on Wordsworth’s prefacing the poem with a quote from Virgil’s Eclogues?  Is that there for what it says literally by itself, or an invitation to read the ode in the light of the Eclogues?  Also, edition of collected poems I consulted had “Intimations of Immortality” in brackets.  Was that title there in the original, or is it something that has been added, as showing how people have understood the ode?
Can I be really wicked and say i don’t like the poem much?
The experiences are suggested twice by using light as a metaphor, e.g. “Apparell’d in celestial light”. And they seem to be stated — or insisted on — by words like “splendour” and “glory”: “splendour in the grass, of glory in the flower;” Is he really doing enough?
There are also lines, and parts, of the poem that seem limp and unsuccessful to me.  I find this line pretty bad:
“A six years’ darling of a pigmy size !”
Here’s another fragment: “The pansy at my feet/ Doth the same tale repeat:”. That seems unfortunate.  That may be partly because of associations the word “pansy” has picked up since Wordsworth’s day. But also here, and elsewhere, It seems to me that there are places where the poet is serving the metre and the rhyme-scheme without serving the movement of the poem.
Compare:
I got me flowers to straw thy way;
I got me boughs off many a tree:
But thou wast up by break of day,
And brought’st thy sweets along with thee.
There I see metre and rhyme hit well, yet I can follow the emotion (informed by thought) moving behind, and the emotion and the form seem to work together.
But it’s not even that.  *Just as a fragment* this line of Eliot’s: “Is it perfume from a dress/that makes me so digress” works for me. I like the feel of those particular sounds falling. I can also understand how a perfume could be evocative, and I think the line is itself evocative, redolent of the feminine. One can imagine the perfume leading to vague associations and memories and derailing a man’s train of thought.
“The pansy at my feet/ Doth the same tale repeat:” What does that say other than “the pansy, too: same for that”?
But I’ve probably failed to justify myself. Is this one reason why people often do OTHER things with poems and prose — “texts” — extract doctrines from them, criticize ideologies imputed to the poet, or whatever?  Because saying why something works, or doesn’t, is really hard.