Kay Lancaster wrote:

Dig a better hole (through the hardpan) and plant a new tree. Russian
olives are not, imho, one of the finer landscape trees out there, unless
you're dealing with really rugged conditions. This one has been badly
stressed, and is unlikely to grow as well as a freshly planted one
in a properly dug hole.

Great stories Kay! Enough to scare anyone away from trees for life!

I agree that R.Olives aren't the finest of tree specimens, however this
one has a bit of sentimental value to me.
And although it has been badly stressed it has shown tremendous
resilience.
The growth that it put on this summer alone is quite remarkable.

It was lying along the ground all through the winter. I was planning on
leaving it in that position and letting it grow upright from there. But
a neighbour came by and was strong enough to help lift it upright
again.

Late last summer when it had fallen over, I started to dig around the
roots to see if I could dig down and make a proper hole. But I quickly
started to hit its roots (which were just a foot under the ground) and
didn't want to risk killing them.

If I were to follow JoeSpare's advice (replant/move), which would
involve digging in many directions for many feet (I'd say at least 12
feet) could the tree survive the cutting of main root systems?

This is one heck of a heavy tree now. 15-20 feet high, 8 inch trunk.
Even three guys now would have a tough time moving it.