Your points are good, and probably more in keeping with the original intent of the story than mine. Obi-wan does say that Anakin was seduced to the dark side, and not forcibly turned in a moment of weakness. It is true as you say that Anakin is very devoted to the Dark Side, I just don't think we need to see that level of devotion twenty years earlier in the prequels. Notice how much darker Vader became between ANH and ESB. He went from simply dealing mercilessly with his enemies to destroying whoever stood in his way, be it friend or foe. It seems to me that if his character could change that much in a few years, tracking that progression back twenty years would show a character who was only on the first steps of his dark journey. However I do think it's important to show that first step, albeit in a way that only becomes clear once the entire saga unfolds.
I imagine the Jedi during the prequels as the last of a noble order, almost gone and far past their days of glory. They are much like the samurai, willing to die rather than compromise their values, which leads to the Yoda-like ideas that one step onto the dark path is enough to condemn an entire lifetime of good works. It's also like Lancelot from The Once and Future King. He has the power to work miracles, but when he besmirches his honor he knows in his heart that it is not his power but the grace of God that allows him to do miracles; he is unworthy of the honor, and this leads him deeper into darkness. Likewise, Anakin must choose whether to admit that his power is tainted, or admit that the Jedi were wrong about the Force. The disconnect is too much for him, and he breaks from the order under pressure of the Emperor. Granted, this should not be stated in such obvious terms, but it's the feeling I'm going for.
Obi-wan is of this old viewpoint, yet in training Anakin he has neglected to tell him all that he should about the Dark Side, for he realizes the absurdity of asking someone to trust their feelings while also making them mistrust these same feelings. In his hubris he believes that he can do better than Yoda and make a better Jedi than he ever was. This leads to Anakin wishing to face the evil Emperor and Obi-wan having second thoughts about his new training method. Obi-wan tries to dissuade Anakin from confronting the Emperor, but Anakin is adamant. It's like the scene from ROTJ where Luke is trying to get Vader to come away with him, and could go something like this:
Obi-wan: "Anakin, please, don't do this."
Anakin: "Before today, I've believed your counsel to be wise. What do you suggest I do?"
Obi-wan: "It doesn't have to end here, like this...come with me..."
Anakin: "Into exile? You have always told me to confront my enemies. Do you think I'm unready?"
Obi-wan: "Anakin, there are things you don't know..."
Anakin: "About the Force?"
Obi-wan: "I...I've failed...as a teacher. I haven't been honest with you."
Anakin: "Don't fear for me, Obi-wan. You've been a good teacher, and a good friend. I know I will always be a student in your eyes, but I am ready. Whatever happens will be my own destiny."
Obi-wan: "May the Force be with you Anakin."
This is the final scene between the two of them, and when Anakin faces the Emperor, the Emperor plays off of Anakin's fears and conjures a story of Obi-wan's previous apprentice:
Palpatine: "Your thoughts betray you, Anakin. Obi-wan didn't tell you about his greatest failure."
Anakin: "What do you know about Obi-wan?"
Palpatine: "In his arrogance, he twisted the teachings of the Jedi to his own devices, breaking every rule of that sacred order to produce a living weapon."
Anakin: "You're lying. Obi-wan has always upheld the Jedi code."
Palpatine: "What do you know of the Jedi code? Yet I see you require proof. Allow me to introduce you to the apprentice Obi-wan doesn't want you to see."
(A dark figure emerges from the shadows of the throne room. The figure activates his lightsaber and attacks Anakin, and they duel, matching themselves stroke for stroke)
Palpatine: "You can see your master's training in him, can you not? Is it clear to you now? You have been lied to, Anakin."
(The figure attacks relentlessly)
Palpatine: "Which of you is the stronger, I wonder? Obi-wan's failure, or his success?"
(The dark figure gains the upper hand)
Palpatine: "I sense fear in you, Anakin. My apprentice, Darth Vader, is beyond fear. He is better than that. Better than you."
(Anakin becomes angry, drives the figure back)
Palpatine: "Good, you have some aggression yet. It may save you."
(Anakin realizes what he has done)
Anakin: "No...I can't..."
(Anakin backs off, realizes that he cannot defeat Vader. He deactivates his lightsaber.)
Anakin: "I am a Jedi. A Jedi. You cannot take that from me."
Palpatine: "So Anakin Skywalker will not serve me?"
(Anakin says nothing)
Palpatine: "So you would die, rather than betray your code? Always the Jedi, even in death. So be it. Henceforth, Vader shall be my servant."
(the dark figure moves in for the kill. He swings his lightsaber and the scene ends)
Obviously Obi-wan has no other apprentice, and Vader is simply an apparition such as in the Dark Side cave. The final scene, which may be weeks or even months later, probably after aggressive mental and physical torture at the hands of the Emperor, Anakin emerges as the cloaked figure of Darth Vader. The torture wouldn't be shown or even implied, but it would be assumed that Vader was a real apprentice who won the fight and serves the Emperor loyally.