Yes 6 gates are blocked while US departures are in operation but
AFAIK EK and EY request the specific stand and I think it best caters for the B777 and possibly doesn't block a second stand. I presume it was the afternoon arrival coming in earlier than scheduled that caused the wait.
I not sure if all the US gates are still blocked once the main bulk of departures happen (ie between 12.30-16.30) but as EI's 330's would be coming in from Europe arund 2 ish they probably still closed off. Qatat wouldn't have a problem if they were to come they would get a stand, something like this is small and wouldn't stop them. They bus passengers to terminals at other airports and as they are willing to use EDI facilities DUB won't have a problem!