please also read point 6 --- a fresh new idea, with also very easy implementation
Point 6 is something 99% of players won't ever use and that 1% that will, will use it only once. Even you, who propose this, will soon get bored of it, especially as you already can watch some of the best players playing live as observer.
If I was in charge in Relic I would never go for something like this even if I was completely sure it is going to take no more than a day for a single programmer. And it never is as simple and cheap in big companies, even if it seems so.
And if you are really so curious, you can always occupy one slot, wait until the game is over and then watch a replay - that way you can get full vision without interfering into the battle of AI. It is also possible that there is a "disable fog of war" mod somewhere but I didn't look for it.