Double digging ground that will eventually be borders, I have access to as
much well rotted [1] horse manure as i like. Two questions, How much should
I add? and At what depth should it be added? I know rotovating it in would
be best, but this aint an opption, just a layer added at some
point........Its hard enough work to start with!

I had been thinking of a 2" layer about 6" from the top.

The ground, I think is a slightly sany loam, that has been compacted to near
a concrete state.

[1] The Manure is black and the straw in most is unrecognisable, their is
little to no smell

How quickly can I plant into this, providing I make sure no manure is
touching the new root ball.