Being unaware that ResMed ventured into the oral appliance field, I looked at a youtube video and it definitely had bit impressions for teeth on both upper and lower jaw. The gizmo appeared to have a way of ratcheting out to extend your jaw or retract as well I guess. If it is just resting on your lower teeth with no bite impressions, doesn't seem like it was fitted correctly. If your insurance paid for this gizmo, perhaps they could interject themselves into the process with whoever billed them for the fitting.
I am a dentist working in dental sleep medicine. Yes, both the upper and lower trays should snap onto your teeth and be held firmly enough in place that they should not come off at night. Personally, I did not use the Narval appliance when they were available in this country. However, my colleagues have told me that they have found that they either fit correctly right out of the box or not and it is very hard, if not impossible to adjust them. If Narval is no longer honoring the warranty, I don't know what recourse you have other than working out some kind of equitable arrangement with your dentist to place an alternative appliance. Arthur B. Luisi, Jr.,D.M.D.
Just for reference, my jaw goes slack when using a full face mask and my mask ends up in my mouth. To fix this issue, I use what is called a boil and bite mouthguard; used by ball players and MMA athletes. This keeps my jaw from going slack and allows my mask to stay in place which effectively aids against leaks. I am not sure why you are using the appliance but if the reason is your jaw slackens when sleeping, I would try the boil and bite mouthguard. You can buy them in a lot of big grocery stores, drug stores and amazon, etc.
I looked back and read your original post here in 2017. At that time you were a new user of the ResMed AirSense 10 and were asking if there benefit in using a fixed pressure instead of an auto adjusting pressure when treating OSA while having issues with atrial fibrillation (AF). My view is that there seems to be very good evidence that treating OSA is likely to reduce the chances of further heart and stroke complications from the AF. I have not seen any evidence that constant pressure is better than a variable range. I think the answer to that would be whichever method provides the lowest AHI. In my personal experience I think one can get a lower AHI with a very tight range of pressures or even a fixed pressure, once you know what pressure is needed. The reason for this, again in my view, is that an APAP is a reactive device, and needs apnea events to trigger a pressure increase. If you have already manually forced a higher pressure that trigger event is not needed. For that reason it may reduce AHI some.
I see now that you have moved to an AirCurve 10 BiPAP machine. It is not obvious to me that a BiPAP would be all that necessary if your treatment pressure is in the range of 12 cm, as you indicated. However it can do everything that an APAP can do and more. It is my view that using a different pressure for IPAP (inhale pressure) compared to EPAP (exhale pressure) can reduce apnea treatment effectiveness, assuming you do not need any respiratory assistance. In my experience you need a higher IPAP to compensate for the lower EPAP, although there are some comfort benefits to a lower EPAP. The net result is a higher maximum mask pressure when there is a differential.
You also mention that you are suffering from clusters of apnea events. Are they obstructive events? If so there seems to be some evidence that they can be positional. In other words you get your head and neck in a sleeping position that is more susceptible to blockage. Some with that issue report having benefits from using a cervical collar. They are not costly and it may take some experimentation to get the correct size and firmness.
Hope that helps some. I have no experience with the mouth appliances, so can't help you there.