Hi, Frank,
The lightest effective blankets I've found are the Berkshire Blankets Softec (BBN). I'm a bit fanatical about blanket weight myself, as I currently use two + two sheets, in alternating layers, since I can't tolerate central heat.
If your shoulder is already subluxated or you've lost muscle in it, you want to be very careful with exercise (holding the joint in its socket or approximating that), but you can still do massage/range of motion during the day to try to extend your stomach-sleeping days. Your clinic's PT should be able to show you this, or if you are eligible for home health, you can get a PT to come out and get you started.
Still, the day will come when that won't work, so you might try sleeping in another position to see what kind of supports you will need (arm, knees, heels, etc.) so you can be prepared. And sleeping on your side or back might help your shoulders either way.
The best pillow we found for shoulder support is the Sleep Better Isotonic Exquisite Comfort Visco Shoulder Pillow. My husband's shoulders were both sublux'd. But he was only able to sleep on his back; thus, my caveat.
I don't know what your lung function is, but if you're waking up feeling like you need to clear your throat, BiPAP could help by ensuring a constant air flow and thus finessing some of the mucus/dryness, in part through humidification as well. In turn, you might achieve deeper sleep stages that would enable you to stay asleep for longer periods at a time. To see how much deep sleep you're getting, you can get a rough approximation with the Sleep Cycle app if you have an iPhone or iPad. I'm sure there's something similar for Android.
Do you have the drooling/stuck throat thing when not in bed?
You can probably guess the next thing I'll say -- might be time for a hospital bed, because when you lie flat on your stomach, your shoulders are just being pulled apart (kind of literally), and having the head elevated usually helps breathing/mucus/saliva flow as well because it is easier for liquids to stay down, if that makes sense. Your airway is a series of valves whose function is gradually compromised by ALS. And while I would normally recommend a foam overlay for any bed to get deeper into sleep, might make more sense to invest in one that fits your next bed, if that switch is not too far away.
Best,
Laurie