Technically, you're not protected if the DL flight is delayed that you miss the connecting EK flight, if you're on separate tickets.

You don't have to exit through customs. You simply have to find the right transit desk to have your boarding pass issued. When you exit the gate, look for the transit desks and find the one that services EK flights. You can even check baggage if you'd like. Just have the DL agent interline the bags when you check in. However, first call DL to verify that it has an interline agreement with EK @ HKG.

1.5 hours is more than enough time. And yes, I've done this before with DL, connecting to PR in HKG on separate tickets.