I am not surprised you are annoyed.
But, you can't send that note! If people cannot be bothered to reply in the first place, they will not be receptive to a stroppy note, they will just gossip about you.
If I was in your position (my son has ASD as well, so I can understand where you are coming from with lots of the stuff you are saying, about food etc), I would go ahead with the softplay (presuming this is a flat fee, rather than per child) and go ahead with a fair bit of food that you were going to make.
If only 5 kids turn up, they will still have loads of fun and 5 presents from friends will be fab. If more turn up, fine, if not, it will still be fine. Worst case scenerio = some wasted food.
My DS is 3.8 so I haven't had to do a proper party for him yet. Previously, I've had the grandparents and DD and a piece of cake and he has loved it. I'm sort of dreading a birthday party!
Just remember that getting stroppy with parents (however badly they have behaved) will not benefit your DS in any way.