OP,
You:
"...trying to entertain DS or keep him from bolting out the door as soon as it's opened to let someone in."
"...it's catching him before he shoots out the door and is off before I can even turn dc2's pushchair around to run after him."
"I do tell him to use his indoor voice."
"I'm literally either playing peekaboo with him or pointing out what the letters on the wall say..."
"He has global developmental delay, and suspected ASD with ADHD traits..."
"...So yes, the games you would typically play with a child of his age most of the time isn't suitable which led to playing peekaboo."
"Actually I do it at home, the shops, on the street, any where and everywhere because he still jabbers 75% of the time, he's classed as non verbal by the speech therapist because you can't understand him at all which most of my mum friends at school know because they've seen me do it walking to and from school, I doubt that's performance parenting"
"I walk down the street singing nursery rhymes, guess I shouldn't be doing that in public either..."
"I'm definitely being oversensitive. Stressed, exhausted and oversensitive..."
Some PPs:
"Maybe they think you're 'performance parenting' and they find it annoying."
(Nope, no mention of quinoa, in Mandarin) 
"Presumably all the other children waiting for the same nursery session are of a similar age and managing to wait quietly."
"...but she can sit him on her lap with a book and point at letters together in that. Or, play a game of hiding a toy under a cloth."
(Surely extra difficult due to his above medical conditions, and OP also having younger sibling in tow?).
"If you let your child "get loud" in an enclosed, indoor space, on a daily basis then they are going to glare, especially if ti seems you are encouraging it."
(Eh? Encouraging DS to get loud how exactly: OP's already said she encourages his "indoor voice", and tries to distract him by interacting and playing games).
OP,
I have worked with many young kids, incl. some with additional needs, but I'm not a parent myself so a different experience entirely.
But, FWIW, sounds to me like you're doing the very best you can, both to keep safe and occupy your DS, whilst managing your younger DC and in attempting to show consideration for others as much as is humanly possible in those circumstances.
Must be bloody hard juggling all that.
Other than only ever arriving at the last minute, thereby missing out on practice opportunities for DS, and (hopefully) camaraderie-building with other parents for you, I really can't see what some expect you to do?
Supportive, experienced advice upthread...I just wanted to wish you (and DS) well x