Rather long history but please bear with it

I am trying to start a veg plot close to my kitchen approx size of plot 5m x
3m
Until last weekend the area was covered in:
2-4" pea gravel on
1-2" weak concrete on
plastic membrane on
levelling sand

Having removed all the above I have found the following:

One end of the plot is dry clay with all the goodness leached out by
adjacent blue lawson firs
Towards the middle the clay is grey/blue and wet
At the other end the clay is wet and sticky

I have dug this over as thoroughly as possible, but need a program to try to
establish a bed that I can work with next spring.

Will this clay actually break down given the right treatment?
My intention is to hit the area with plenty of mushroom compost and
pelletted chicken manure.
The area is at something of a low point in the garden, so I'm not surprised
that it is damp particularly considering how well it has been covered (my
guess is that this arrangement has been in place for at least 10 years).

If I use the mushroom method (and it works) and then add 4-6" topsoil and
more compost, I reckon I can get a workable bed depth of around 12". Will
this be satisfactory for most vegetables?
Can I add fresh (non-rotted) horse manure at this time of year and expect it
to break down and do some good by the spring? This would be additional to
the miushroom compost.

Finally any further suggestions for improving this area (I am reluctant to
excavate any more as it is back breaking and a difficult area to access with
serious machinary)

The plot is in a north-east corner and gets sun from about mid day to 6pm

Thanks in anticipation from a mega newbie would be gardener

Phil