So one of the painful lessons I picked up in industry is this: an impossible deadline plus a great desire to change is still an impossible deadline. Scheduling is not my bag, and I'm far less well acquainted with Diaspora than you are, but if I was four man months from release at the day job with the state of the project where I think it is I would be sending out emails saying "We will not hit this. It is impossible. We need to cut scope or push back release." to my superiors.

I don't know if you can get a security expert to fix this for you. You can certainly wave a big enough check around to get somebody to look over your code, but that won't magically improve code you haven't written yet. Also, it is highly likely that Diaspora is architecturally insecure -- that, beyond the "Oops, didn't check the input" code-level whoopsies that your federation strategy as written (and apparently as not documented outside of the sourcecode) just cannot be made to work right.