I'm looking at this from a PM's perspective and not a devs perspective so take it as you will.

>he basically tells me this project should have been finished a long time ago and he can't believe it's taken this long >>Sounds like a manager with a MBA and lack of hands on technical experience.

>he says I probably just misinterpreted an offhand comment of hers as a hard requirement >>The problem isn't you not understanding her requirements, its a lack of documented well defined requirements - and any changes to said requirements should be noted and tracked.

>no longer necessary due to changing requirements >>Once again, this sounds like your manager is deflecting the fact that they didn't have well thought out and well defined requirements before the project began.