Drainage and size have indeed been cause for some concern, if not a slight headache. (Thinking ahead, I worry that "I drowned a tree" might not be acceptably dull content for this community.)
However, the last hole I dug for another tree not too far away from this present hole turned out fine, so I hope the ground is okay. As suggested elsewhere, I'm also filling the hole with new, extra dirt.
I... yes. Yes, I will use my money to buy new, better dirt to replace the available dirt I painstakingly dug up and carted away to a slightly different place.
Thanks!