@zizaspeed; as to if it will stay open or closed, I honestly don't know for sure as I'm not keen on the internal workings of the EGR. However, I was able to block off the non-fittable EGR bung on the header, i just stuck a bolt in the hole that was just big enough, and clamped it shut with the EGR tube holding it down. It honestly does a perfectly fine job, no (noticeable noise or smell) exhaust leak and not moving/rattling.
For your rattling, were you able to fully secure the header to the rest of the exhaust? (where the two bolts are at the end and have it hook up to the donut gasket)