The Boy ended up resolving the large mailbox hole issue by inserting and affixing a large chunk of wood in the hole and painting it with Drylok. Then he hand sculpted a faux brick (with mortar lines and everything) out of cement patch.
Now we just need to seal it with our exterior mortar sealer / primer and paint the entire exterior of the house. But first we have to decide on an exterior color.
He also drywalled and mudded the interior wall to close that big scary gap. Hooray! It’s a solid wall again.
BTW: It’s already painted. (As is most of the guest bedroom)…oh no, the suspense…