Unfortunately, this is post not accurate.

As with your upgrade example, premium lounge access is determined by the fare paid rather than the cabin you are sitting in. Because there is no business cabin on those routes, QR do allow business fare pax connecting to intra-gulf flights to sit in the F cabin, and also to use Al Safwa IF their business class fare is eligible for lounge access. But if your ticket would not get you into Al Mourjan (ie. you bought a (P) fare), it will not get you into Al Safwa either. The only exception to this is for QR Platinum members, who are granted access to Al Safwa regardless when travelling on a business fare.

Your confirmation may indeed say that your cabin class is first, but right next to that will be the business class fare basis. And as if to prove the process, all passengers will thus earn points etc based on the fare basis paid and not the cabin they were sitting in.

The technical explanation for this is that when you buy a ticket from say OSL to AUH, you are buying a single fare basis between your origin and destination, not J to DOH and then F to AUH. You are offered a choice of Y fares throughout, or J fares throughout. And if you departed from say LHR you would also have the option of F fares throughout because F is operated on both sectors.

You could potentially buy separate cabin fares as a multicity itinerary or separate tickets to bypass the (P) fare restrictions (that may actually be the only way to arrive in Y and depart in F), but you would then be paying for an actual F ticket DOH-AUH which would be much more expensive (similar to how separate redemption tickets cost more, particularly if booked in different cabins).

Back in the day, until c.5 years ago, you could book cash tickets with a choice of cabin on each sector and receive points accordingly, but that is long gone. Other airlines may sell their tickets differently, but for many years now on QR you buy a single fare basis from origin to destination, irrespective of multiple sectors.