I think, us is simply an objective pronoun as is me. It should be "let's, you and me', not 'you and I'. An appositive should be of the same type. We can, for example say: "We, you and I, comment on this point."
In the same way, we say, She asks us, you and me.

In Eliot's lines, the use of "you and I" is just to rhyme with "sky", and this is possible in poetry.

All the best
ajkhalili@yahoo.com