I deleted some of the posts here, since the debate shifted out of topic we have at our hands too much. Please try to elaborate as much as possible while not going out of subject.
This theory might be a bit far-fetched. Art-wise, they are similar, but that's far from being a proof.
That said, we aren't even sure if Ikumi showed a reaction towards Isshin's presence or not, though. So, we can't rule it out by that argument. Also, this is a theory and theories do not really need solid proof to be thrown out.
And the argument is not really that sloppy, meaning, Kubo can make that connection in the future if he wanted to do so. Having a Quincy to watch over Ichigo isn't really a crazy thing to do. And for all we know, provided they have a connection, Ikumi and Bambietta do not have to share and pursue the same ideals. So, Ikumi might just have switched sides to protect Ichigo, as well.
So, yeah, we can fit this in somehow. I don't think it'll be the case, and the story will be kept simple in terms of the past stories, though.