Hey Grant,
Yes, I think that your time line would be pretty close to right. When each company discontinued 1/2 track production depends on how they implemented 1/4 track distribution. Most of the majors (and Columbia did their tape production in house) continued making 1/2 tracks while they were still "testing the waters" with 1/4 track.
The original mono lp of KOB that I have is dated 1958 and I believe that was when it was first released. I'd have to check and see when the first stereo release was but I would bet that it was very close to this tapes release date.
I seriously doubt that a TP release of this or any other highly sought after titles would have a whole lot of effect on the prices that some of these things command. Once an item shoots into the price stratosphere, I think we can safely assume that it's entered the realm of the collector and as far as sonics per dollar go,.. all bets are off!
A TP release would probably just add another item to the list of releases that a serious collection of ,.. in this case, Miles, must have.