I am with you up until the memory stuff. Roxas was fully functional, and wielding a keyblade before he had any of Sora's memories. There is a period (training period) where both Roxas and Sora are awake, both wielding keyblades. He only started gathering those memories as they seeped out, due to Naminé's complete reconstruction... which occured on [Day 26] . Due to this, Roxas also falls into a bit of a coma. So either I am mistaken, or your memory theory doesn't count for the 26 days Roxas wielded a keyblade without Sora's heart/memories.
First, that isn't true in Roxas case. The ability to wield a keyblade is tied to the "heart".
Let me be clear. Sora's heart is in Sora. However, the memories which make up a heart are necessary for a person to function. Thus, Sora's sleep-state at the end of COM and also Roxas sleep-state at the beginning of 358. Nobodies usually get the memories from the heart that left the body, however Roxas is different because the two hearts used in his creation aren't capable of sharing memories. First, Ven's heart is inside of him but is incapable of giving him memories because it sleeping. This is a vital reason he can wield a keyblade. Second, Sora's heart which resides in Sora so it can't give memories to Roxas. That is, until the his memories get jacked and he has to sleep. During this period, many of Sora's memories were tied up within Roxas. This is why most of Sora's heart was in Roxas.
Another point; Xehanort is a Keyblade Master. Terra is a Keyblade Master. Yet Xemnas can't wield (that we have seen.) Nor can the other nobodies from the Org. that have been given Xehanort's heart. The only 2 versions of Xehanort that can wield keyblades are Master Xehanort, and Young Xehanort. These are THE SAME PERSON from different times. What was missing in these? "Xehanort's Memories."
Memories are required to form enough of a heart to wield a keyblade. In fact, in a riddle, Nomura says that "the keyblade that Roxas wields, and the thing that Sora would lose in Castle Oblivion are the same." Clearly, that is saying that the keyblade the Roxas wields is Sora's based on the memories of Sora he has. (Btw, it should be noted that Roxas has always had a small amount of Sora's memories from the short period of time that Sora was a heartless, thus explaining his ability to wield.)
Regarding Xion, she isn't a Replica of Sora, she is a Replica of Roxas and is given form due to Sora's Memories of Kairi. The fact that she appears inside Sora's heart at the end of DDD is evidence that she does have a heart. When she wields a keyblade, she is using the one made from Sora's Memories. She was absorbed into Roxas.
I'm sure i missed stuff. Call me out. I love this crap :P
Moving on to Xemnas wielding a keyblade, Nomura once alluded to the fact that Xemnas can wield a keyblade, he just doesn't want to.
Xion being inside Sora isn't confirmation that she has a heart. It's just an acknowledgement of what Naminé spoke about near the end of CoM: That memories are never truly lost, even when erased, and continue to exist deep in the heart.
-- Roxas, the "Sora + Ventus" Nobody, was able to use a Keyblade. In contrast Xemnas, the "Terra + Master Xehanort" Nobody, wasn't able to use a Keyblade. Why is this?
Nomura: I'd rather that point remain a mystery. It's possible that he intentionally wasn't using one.
Also, I don't recall Xion being confirmed as a Replica of Roxas, but nor do I recall her being confirmed as a Sora replica either. So I will take your word for it.
However, I believe her Dream Drop Distance journal entry should read:
Confirming that she was absorbed into Sora, not Roxas.
Organization XIII's mysterious fourteenth member. She was an experimental replica used to "catch" Roxas's memories and create a Keyblade wielder for the Organization.[spoiler] In the end, she was erased from existence and returned to Sora where she belonged.