Ooh, what a fun little idea egg!
Let me see...Marluxia would probably have to rework his plan a bit, but we know that Kairi's princess/light powers didn't prevent her from 'forgetting' Sora via Sora's altered heart state, therefore Namine would just as easily be able to manipulate her as anyone else.
However, Kairi seems to have a knack, as it were, to kind of see peoples' 'true selves' despite their physical appearance. (Sora's heartless and to an extent, Xehanort-Riku-- albeit being a bit surprised at first, she very quickly figured it out). I'd imagine she would actually be pretty spot on at figuring out that Repliku wasn't real Riku. We know that the connection Sora and Kairi has is very deep and it took Namine a lot of chipping away before it finally got covered up. Sora met Repliku way before that point, so unless Namine hyper targeted Kairi first (which I'm not sure would have been possible)...she's going to quickly figure out something is up. That would throw a huge wrench into Sora's early 'motivation' to keep going deeper into the castle because it would be a bit obvious something is up.
Presumably this would then set into motion a few things. It would rightly establish that Kairi is a threat to Marluxia's end goal and if he hadn't already decided on it already, would extra motivate him to target her.
The only reason Marluxia likely never bothered with Donald and Goofy (and Jiminy I guess) is because they wouldn't really be much of a threat to stopping Sora from prolonged exposure to Namine (aka staying in the castle), but Kairi could potentially dismiss any/all of Namine's early lies because she was actually there. Though, I'm not sure the extent that Namine can construct fake memories in other people, but maaaybe she could plant the same memory in both of them like she did with Repliku?
Conclusion: unfortunately, unless Marluxia is very dense or Namine is greatly overpowered, i'd bet Sora and Kairi would be getting separated again at some point. Either Marluxia tries to kidnap her or whisks her away during whatever the hell kind of magic turned Donald and Goofy into cards. If they were together, I doubt Namine would have as much hold over Sora's memories/thoughts of her. That's the only way castle oblivion would be able to play out similarly.
Though I wonder if Marluxia would just consider trying to assassinate Kairi right off the bat. I mean, the whole reason he is even using Namine in the first place is because he knows Kairi = Sora's light. If she were to die at this point, presumably Sora falls into the darkness having lost his hope and would be easier to manipulate...
But that's dark and I don't wanna think about that.
My other major consideration would be if/when Kairi would get access to her Keyblade. That's still a bit of a weird point even in canon, but we know that theoretically she should be able to use it at any time so I suppose she could technically figure out how to pull it out earlier. If that happens, perhaps Marluxia would actually target Riku instead?
Rather than just have one puppet, why not try to have two keyblade puppets, right? And both Kairi and Sora would be motivated to try and help Riku so maybe instead of dangling 'new girl Namine' out in front of them he would actually try to sniff around the basement of the castle to find Riku and/or get a new angle that way.