I think the big difference is that you tried to keep ds under control. In the thread you're talking with the ruined lunch they didn't do anything. It also was a very expensive restaurant so probably not where you would go if you were struggling with debt - although obviously I could be wrong if your family paid for example.
You need more support though, it sounds very tough and like you are quite over whelmed. Can you and your OH split your time at weekends so each son has some one to one time with each of you? Can your son read to both you and your other son while you all snuggle in bed in the morning perhaps? Is your son with ASD constantly overwhelmed as well? Would ear defenders help if he doesn't have them already when you're out? An OT might have lots of ideas of things you could put in place to help him. Routine is critical - same routine every day after school is ideal, if he has an activity then I'd start the routine as soon as he gets home. I'd also keep activities to a minimum, ds is much worse behaved if he is tired - also snacks between school and activity, hunger often also leads to coping less well. Routine as much as possible at the weekend too, calming and quiet with not too much going on may be best although a good walk and time at the park might be important too if he has both ASD and ADHD. A plan for each day at the weekend might help him, so he knows what is happening and what is coming up next. The other thing that he is really important is his own room, having his own safe space is vital IMO (he might already of course). Maybe you know all this already though, apologies if I'm just saying things you already know.
For you OP I tihnk you need to tackle one thing at a time, I know how easy it is to feel over whelmed when you are ND. So maybe make a list of things you need to sort out/people to contact or whatever and think, this weekend I'm going to tackle the first one and concentrate on getting that one thing started/sorted and not worry about any of the other stuff. Then each weekend give yourself one more thing to look at either with DH or take turns whatever works best. Do you have anyone who is willing/able to have the kids every so often so you can just have a break/night off? I really think you need something like that if you don't already (but may not be possible I know).
One thing - you say that you don't like ASD as you don't see autism as a disorder but use ADHD where the D also stands for disorder. They're both neurodiversities, both medically considered disorders and the number of people with autism doesn't affect whether it is a disorder or not, disorder really just means different from normal. There are around 700,000 autistic children and adults in the UK in a population of 67 million so people with autism are still very, very much a minority.
Good luck OP, I hope you and and your dc get more support and the school are really good as the last one sounds rubbish (a lot of teachers don't have a clue about ND's though unfortunately and are completely over whelmed themselves). Also I wouldn't rule out a residential SEN school for DS in the future, he is already hurting you and this will only get more difficult to handle as he gets older. It could be a really positive thing for both of you as they will be able to provide a very structured routine for him that just may not be possible at home. It also gives you a break and would allow you to concentrate on your other ds. I would also look at young carers for your other DS I don't know much about them but it could be a good positive thing for him as it sounds like he misses out due to his brothers behaviour.