Re: Registering with another users email ???
Well login integration is not as important as the register part. As far as your instructions, I wouldn’t recommend deleting the contents of a core file but the redirect part is okay.
It occurs to me, it’s actually possible to redirect the register page in bbpress to the wordpress side via plugin with no edits required anywhere.
Remember that bbPress registration process should catch up to the WordPress side eventually. In fact I like it’s layout better but WordPress’s is more mature code-wise.