Knowing table placement (glass to the right, side plate to the left) is useful because it ensures that everyone has their utensils by them. I have had a couple of times at a formal dinner where a person next to me has taken my glass/ plate and left me asking for one halfway round the table.
Generally I'd wait for everyone to be served before starting to eat. If there is a delay on a choice being served then someone will encourage those that have been served to eat while it's hot. It's better if people are eating at the same time.
Ettiquette on fork- left and knife-right, is assuming a NT, right-handed majority and does avoid elbow banging of people cutting elbow to elbow. Trying to train a ND child how to eat effectively and with minimal mess is interesting. I don't expect perfection and it's very much pick your battles (dyspraxia and ASD are a tough combination) but he does need to develop a style that compromises comfort, with practicality and not result in chasing food around the tablecloth or using his hands.
Most parties I go to are informal. You might bring a drink for the host and leave that for them, but your own drinks for consumption, can come and go with you. We provide wine, basic range of spirits and soft drinks/ mixers, and say to bring anything fancier. Our friends tend to be into real ales which we don't drink so leaving those with us is a waste. We went this way after a few parties where we bought beers that no one went for and it was a complete waste.
My bugbear is buffets/ communal serving where you're left with an inadequate amount of food because of people that swept in promptly and took large portions. If everyone takes a modest portion, there should be enough for seconds and everyone to have a decent amount.
I got caught out at a wedding hog roast when 4m postnatal and struggling with my pelvis after a long day and stuck with a ravenous BFing baby hitting clusterfeed mode. I waited for the queue to calm down and there were barely scrapings left. Not sure if it was portion control or people having seconds. I was starving by then.