1: How would Obi-wan be made to acknowledge that Grevious is his prior student? I suppose some introduction could be made during the crawl since that is also the case in the theatrical version, but it would need to be buttressed by dialogue from Obi-wan confirming their history. This would have to happen early in ROTS, since Obi-wan could only have taken another apprentice during the Clone Wars after Anakin had become a Jedi Knight.
Actually, I was planning to have Obi-Wan mentor Grievous before TPM: in my planned saga edit, it will be established that Padawans can tutor other Padawans in a Japanese senpai/kohai sort of fashion. And I don’t think it’s possible to have Obi-Wan mention a previous apprentice, but I was planning to add some redubbed lines from Yoda hinting at it. For example, after Qui-Gon says that Obi-Wan is ready to take an apprentice, Yoda says something like “Your Padawan had an apprentice once. Know how well that turned out, we do.” The teases would get more explicit in AOTC, until Grievous is finally revealed in ROTS.
2: If Grievous is already so far gone as to have a metal cranium, it would be pretty clear that this is not the character in the OT when we see the back of Vader’s head in ESB. There’s also the problem that Vader in the OT and Grievous in the PT have entirely different ways of moving, with Grievous being much more alien and animalisitic in his movements.
One, I’m planning to edit the OT too, and I already knew that I would have to cut out that helmet bit. And two, I feel like Grievous being more alien and animalistic could be explained by him, well, having an alien and animalistic body. It would be implied that after Grievous’s apparent death at Obi-Wan’s hands, his robot body was upgraded into a more lifelike, human form. Of course, this wouldn’t turn out to actually be the case, but still.
3: Obi-wan in ANH says that Vader betrayed and murdered Luke’s father. However, there is no one-on-one fight between Grievous and Anakin, and Grievous dies in act 2 long before we see the last of Anakin. If the film is cut to imply that Grievous survives, this would also necessitate Anakin surviving to be killed later by Grievous, but this introduces many more problems as well as leaving unclear how Grievous would ‘betray’ Anakin if he was already a darksider when they met (this is a problem with my latest Prequel concept as well, but it could easily be fixed by having Obi-wan’s apprentice meet and befriend Anakin earlier in their adventures).
I thought about that too, but I figured it would be fine if the audience already knows that Obi-Wan is lying to at least some degree. There are some edits that already do that by having the “reveal” in ESB be that Anakin survived the Mustafar battle and became Vader, but I think my idea is an improvement by allowing Vader to still be described as Obi-Wan’s apprentice rather than just some generic mook.