I think you are overthinking it and it will all depend on conditions on the day. The first thing to say is that 90 minutes is typically plenty for a connection at DOH. Most flights arrive on time or early, if you are in J and have a bus gate, you'll also have a coach just for J pax, so won't need to wait for everyone to disembark, etc. Once you have made it past security and into the departure area again, their is transit towards various gate areas so you never really need to do long walks if you do not want to.

Having said that, if you are worried about having to go fast, I'd recommend asking for assistance, there is really no shame in it, it is free, and basically you'll be picked up in some of those little cars and also get a specific lane through security. The main "cost" is that you'll be ask to stay onboard till others have disembarked on all the flights so it may slow you down a little upon arrival(s).

If the flight is delayed in such a way that you miss your connection, then you'll need to see with QR what they propose on the day. That will depend on the time you actually arrive, existing nonstop or indirect options and which ones have seats available. if you are delayed overnight, then yes, they'll provide accommodation etc, but it's equally possible they'd fly you shortly after your initial flight time with a connection somewhere either in Asia - e.g HKG or SIN or in Australia e.g. via PER or SYD) but again, there is little point in trying to double guess it by now as it depends on too many factors.

If J is not available, they may offer you a downgraded itinerary but you would not have to accept it and may choose to fly on a later itinerary where J is available assuming that there is one.