My DS was a terrible napper, too, and you had to perform the magic ritual of the boob exactly right to get him to drop off. I remember standing in the dark loo with the fan on rocking him in my arms, sobbing, because he wouldn't nap.
As my friend said then, sometimes they just won't and there's nothing you can do about it. It's awful, but it's true. My only advice is that airplane thing: "put your own mask on before assisting others". In this case, make sure you're fed and watered and have had a pee, and then worry about the rest. He might scream bloody murder while you're cramming a bite into your mouth, but if you can see him and you know he's not hurt or scared, it's fine.
As for co-napping, agh. It's frustrating to lose all that time to just being a warm body, but I found giving DS a dummy took the pressure off my body, and having a phone loaded with podcasts to listen to and websites to visit helped me cope with the time I had to lie down with him every day. Eventually I started to be able to sneak away for longer and longer stretches.
(Somehow, aparently on his own, my DS has become a champion napper. But sleeping through the night is another thing all together…)
Good luck! I hope you get naps soon!