I have pond apples, lots of them and some are fruiting right now so I will have seeds for you whenever you want.
Some of us experiment and try the seemingly impossible with little expectation, just the pleasure of trying. I am sure I have had my fair share of exercises in futility, I enjoyed the journey and nothing was hanging on the success. If you wish to experiment, go for it. Very likely you will confirm what all these folks are telling you but you will have fun doing it and have those moments of hope and suspense just before what look like a rare thriving selection from your dozens of seed(ling), keels over and prove the naysayers right, again and again. Don't let anyone stop you from tilting at windmills, just have fun doing it, and keep us informed.
Like I said, I have seeds and I will be glad to share them with you.
For me, I have found that they hold Cumbe, Cherilata, Soursop and Hunucma amarilla well. My Ilama on pond apple is a little anemic. These are all recent grafts and there may be delayed compatibility issues but for now they are going well. I am in Florida and pond apple grows like a weed (well) here, so long as you have wet conditions.