Oh good, the first pissing contest of the day, whoo-hoo.
Yes, I suspect he has vanished, you can speculate until the cows come home, but as of now, he has not returned.
As for all your other points, yes, some people are capable of learning new things and buying all kinds of tools, etc. However, if he had the motivation required, it is my opinion he would have been participating in the discussion beyond the initial post and sooner than this point in time. It seems many people who post through the home owners hub, never follow-up.
Also, the idea of cutting a slot and installing a spline seems needlessly complicated when one can install dowels using a jig, a pencil, a drill, a mallet and some glue. The foundational principles of proper engineering include keeping solutions as simple as possible while attaining the desired end result.
Lastly, considering the door is 1.38 inches thick, with masonite outer cladding and a particle board inner core, cutting a 3/4" slot in the inner core would either eliminate the particle board or dramatically weaken it. Not only does a long slot seem pointless, but it would likely further damage the remaining structural integrity of that end of the door. Removing as little material as possible seems to be the most logical approach when trying repair this kind of a cluster-f*ck.
Did I satisfy your need for pissing contest? I have horses to feed and a solar array to pressure wash.