I'm thinking... What if what actually killed Ichigo's mother, Masaki was... Ichigo himself? With masaki having the hollow already within her, and having passed it to Ichigo, Ichigo's inner hollow go full berseker, (without wanting), to avoid that, Masaki "kill" that part of Ichigo temporarily. What do you think?
I started to think exactly the same thing. TBH i think this makes perfect sense to why Ichigo does not remember what happened "That day". Ichigo was way too young to control his powers, if any powers at all and the additional souls within his body. So with what you theorized there it would only make sense that when Ichigo was attacked by the Grand Fisher that he may have actually been attacked by it and injured, went berzerk, scared the Grand Fisher off, Masaki intervening like Urryu did when Ichigo went Vasto Lorde Berserk Hollow on Ulquirra and getting attacked by him, however with Masaki intervening would leave her fate open to 2 thoughts. Ichigo either did kill her or she is still alive and took the chance to disappear and come back when Ichigo was much older to control his powers.
I myself wish to believe Masaki is still alive. But both ideas seem quite plausible at the moment regarding Ichigo's developement.
As for next chapter... or chapters... i want to say... Urahara actually gets involved with Isshin and Masaki and helps create Ichigo as part of an experiment, which Aizen finds out about through time. Since it started with Aizen's work thats why he seems to take credit for it, but Urahara actually finished it. I actually want to say from an old theory i shared with others, that Ichigo was created in attempt to bring everything soul power back into one, to become another Soul King type being. I only say that because of when we saw the soul kings eyes, he had the same diamond pupils with the 4 black compassing triangles like all the captains houri's, which i think represents the 4 soul spirits of the Bleach universe. The Soul King was the first of them all to merge all 4 and succeed.