Hmm...
Grand Fisher did say he killed her. He even has her 'form' which certainly suggests he at least encountered her in some way so as to know how to create her illusion. But...
But there's more, because when Grand Fisher used Ichigo's mom as an 'illusion' there was a reaction, something Fisher didn't seem to be controlling, almost like her soul interacting (on its own volition, despite being 'inside' of Fisher) with Ichigo during the middle of that battle. This highly suggests that Fisher indeed was responsible for Ichigo's mom's death.
Also, Ichigo's mom's soul being a part of Fisher, the Hollow that took control of Tatsuki's body (while she was still alive), and Kaien's soul still having sway over the Hollow that posessed him all suggest that when a Hollow consumes a person they actually become a part of that Hollow. Hollows have already shown the ability to 'absorb' Souls, both Pluses and fellow Hollows (Menos Grandes).
Assuming the above idea to be correct, that souls 'consumed' by Hollows become a part of that Hollow, The question is then this: When a Hollow is 'killed' by a Shinigami does this enable the once-Plus souls inside of it to go to Soul Society? Or what?
I also believe Ichigo's mom has a role to play in the future, either through his dad, in the final encounter with Grand Fisher (an obvious Aizen creation) or just as a continuing source of motivation for Ichigo.
edit to below- sounds like my example has already been solved in the manga then... goddamn i can't wait for Bleach fillers to end...