"Why is that decorating a portion of the wort doesn't kill all the enzyme activity for that portion of the wort even after being boiled for a long time?"
I haven't decorated any wort, yet. But, I am willing to learn.
Yes, enzymes become thermally denatured. But, who cares? The majority of enzymes are in the mash tun, not in the decoction kettle. Besides, when decoctions are correctly performed, the decoctions are converted before being brought to boiling. It has to do with the "Oh Alpha ammalyse happens between this range, and Beta ammalyse happens between this range", thing. Controlling enzymatic action, determines what the final product becomes.
The purpose of boiling mash liquid is to stop enzymatic activity. The boiling liquid is used as the final decoction.
"I feel both of the processes are both active at least to some extent outside of those ranges if for no other reason the fact that mash tuns do not hold exactly even temperatures."
That's why the English method works. Dump hot water at a certain temperature on malt and wait for something majical to happen. The mash tun thing isn't always true. Mash tuns are well insulated, some are direct fired. Mine is direct fired and holds temperature without a problem.
Noonan wrote a good book on brewing. He covers all of your questions.