I agree with basically everything except woth only changing the difficulty once during gameplay.
It should always be entirely up to the player himself to judge. I simply don't see the reason for an arbitrary restriction on how many times a player should be able to switch difficulty. If he chickens out and starts playing it easier than his capabilities allow, then it's his own loss.
The alternative is that a player could lock himself in a difficulty that ended up too hard for him (either because he misjudged his abilities, or because the difficulty curve of the game is a pile of poo, which ultimately isn't the players fault). And then instead of being able to go back to an easier difficulty, he will either resort to cheating or simply just lose interest because he keeps dying again and again and stop playing the game. Hell this could happen the other way around too, that a player locks himself in a difficulty that is too easy, and he also loses interest and simply stops playing because he can't go back to normal/hard mode.
You should remember those feeling yourself, based on your reviews of Demon's Soul (hard) and also Dante's Inferno where you went to easy-mode and were suddenly playing... what was it again? Baby's first vagina adventures?
I'm one of the players who appreciates playing games on easier difficulty settings (i never even played Half-Life 2 on anything than easy), mostly because I'm in it for the experience itself. But even i sometimes appreciate challenge and managed to beat Mass Effect 2 on Insanity without chickening out at any point, even though i did die quite a few times. One of the reasons i didn't chicken out was that the difficulty curve was "okay" (besides the fact that upgrades are too powerful, which makes insanity a pain in the ass in the early game, and actually too easy at the end of the game given it's the fifth and final difficulty setting). Most players who want a challenge can actually appreciate difficult games if designed well.
But at the end of the day, there simply are too many games out there who not only may have terrible difficulty curves, but also terrible difficulty tunings. One of them is Crysis. On Crysis you have 4 different difficulty settings, but in fact, for the most part they don't change the difficulty as much as they change the gameplay. The higher you put the difficulty setting in Crysis, the more it changes from "Shooter" to "Stealth Shooter" because the only way to survive is to snipe all the enemies from cloak mode, something which could have been quite fun, but in Crysis simply just reduces the pace of the game for the worse of it. Luckily you can modify the difficulty settings in the .ini files of the game, and that way make the game difficult while still maintaining it as a pulse-pounding action game, but it's really annoying the devs couldn't get that right from the start.