If you go into your booking on QR.com it will tell you which lounges youre eligible to use assuming your QR Platinum/ OW Emerald is logged against your booking.

My understanding is Al Safwa access will prevail as you'll be arriving in First which books into A/F and you'll be gaining lounge access on the back of that flight inbound to DOH rather than the P class sector onwards which would restrict you from accessing Al Mourjan