Well youre still well within the predicted approval times for EAD, so theres nothing you can do at the moment except wait. If, after 90 days you have still heard nothing, make an infopass appointment to inquire about it.
Like I said in my previous post, the average time for EAD approval is around 60-90 days, and youre not at the 60 day mark yet so youre just going to have to be patient.
As for whether you will go to interview first...well that depends on how quickly they process your AOS papers. I have seen some people have their interview before they get their EAD (and therefore dont need the EAD as they had their greencards approved first), but more commonly the EAD is approved much quicker than AOS.