It all depends on the trees you grow. Many trees, like Japanese maples, would love morning sun and afternoon shade so the back of your house (facing east) is good. Some trees (Japanese black pine for instance) will like full sun. If you can create another bench in your back yard that's not...
For several years I collected coast live oak acorns and planted them. Some years nearly everything sprouted. One year I got only 1 to sprout, others in between. Just can't tell about acorns.
The hole will probably close up eventually, assuming it doesn't collect water. If it does you can create a drain with a drill. The dead spots can be left alone, or hollowed somewhat to form a uro - a feature instead of a flaw. see https://www.bonsainut.com/threads/uro-advice.255/ They'll...
This is all about your vision for it. If you're in doubt, let it back bud and see where you get more options. Personally, I'd chop lower, maybe like this to start:
Pardon my ignorance. Why does one drill a will twice as deep as the water table? To get the flow rate you need? To guard against drop in the water table?
First, put your general location and/or climate zone in your profile. It will help you get better advice.
You've got a nice starter plant with lots of potential. Job 1 is to keep it alive. #2 is the artistic part.
Depending on where you live it's probably a bit late to repot this spring...