Andy here is my 2-cents worth. I think your location is really not a problem. I have 3 espaliers right up against the house near the concrete foundation. (And some others that are not espaliers but which are near the wall.) Yes it is true that concrete may produce more alkaline soil, but you can control that, and I have also noticed (1) a firm concrete footing doesn't leach much, and (2) the camellia roots will take a direction away from it, and (3) the camellia roots are nearer the surface than your footing. Plus hey, sometimes you have have to go with what you got! And what wall these days isn't built on top of concrete slabs or footings?

My question is about that 10 inch hole. Unless you have a big root ball, I would suggest planting it less deeply (maybe halfway in the ground) and mounding some soil mix broadly around it, with mulch on top of that. A common mistake is to bury the root ball, or planting it too low that it eventually sinks below ground level. That is a problem, especially around a roof line where drainage is very important.

And finally, as to form, an espalier camellia can be anchored to the wall if you want to, but I find it easier to simply keep it pruned in a 1-dimensional (flat) style, a few inches from the wall. It is a matter of taste in what you like. Mine seem happier, and its a lot less work, as the camellia limbs are strong enough to support themselves.

So I say go for it! Plant your espalier high (above grade) with lots of good soil mounded around it to form a small berm, and angle the plant so that it is about 3-4 inches from the wall, and then simply keep it pruned and trained to your taste. By the way, I saw a mature one last week! It was a 20-year old Mine No Yuki (white) against a dark brick wall -- stunning.