I would say something, but that just me, being disengaged doesn't mean you have to tolerate annoying behavior.

"SS swallow your drink, no need to swish it around to activate extra plaque bugs in there, you already have plenty."
"SS you will not waste food in this house, go re-heat that for lunch!"
"SS stop talking like you are 2, you are 10, do you talk like that with your friends at school?"

I would love to know too!

We don't let kids walk away wasting food though. It's quite ridiculous that we sometimes still have to check SS13's plate to make sure he eats it all (because otherwise the food goes into the sink and I would become the one ends up cleaning it), but when it is told to him to finish it up, he would grudgingly oblige.

DH is the real problem here. Work on him setting more limits on his kid. Otherwise, I'd just walk away and not eat with them if he's not going to teach his son reasonable behavior around food. 

I'll tell you, I wouldn't be able to deal with that mouth swishing thing.  I am super sensitive to sounds and I swear, certain sounds make me want to murder someone lol.  My  SD is a gross eater.  She moves her mouth in a really odd way that causes her to make a lot of noise.  We never eat without music playing.  It drives my husband crazy but if there's no music I'd have to eat in another room.  She also bangs utensiles against plates in a way that makes very loud noises.  I've started giving her plastic utensils and plates to avoid this issue lol.

A 10 year old is way too old to be baby talking.  Your husband really should stop giving it one ounce of positive reinforcement. 

As far as the food goes, do you contribute to the groceries?  If you do and it's your money that is being wasted on this I would be very mad.  Your husband should tell him If I make this and you don't eat it, I will not make for you again for quite some time.

I have misophonia and that would drive me crazy.

You need to disengage during mealtimes.The SSs used to have worse manners than pigs at a trough, so I stopped eating with them. In fact, I disengaged completely from meals. DH had to do it all: shop, cook, clean up. Once the kitchen was clear, I cooked something delicious for myself. Or I went out to eat. 

Until Daddeeee enforces etiquette, you make yourself scarce.

If DH wants to cook and let it get cold.  I would leave it there for him to clean up.  He cooks. He cleans. Not leave all of this to you.

Yes. DH is the main problem 

1) If you don't feel you can ask your SS to stop, I'd mention to your DH nicely how irritating it is to you and most likely is to other people as well, like when he's eating at a friends house, so he should probably talk to his son and tell him that making noises when you eat is f*cking annoying! lol If it doesn't stop I'd just avoid mealtimes with him. I don't sit with my DH and SK'd most nights, even tho I cook for them, because I can't stand their dinner table mannors, and they're not even that bad lol But if it annoys me, I just try to avoid it. 

2) DH needs to tell his kid to clean up after himself. If DH has and he's not listening, I'd put that plate full of food on his bed next time.

3) Baby voics drive me absolutely insane! At 10 years old that needs to stop. I would not be able to hold myself back from saying something, and I have in the past when SS's and SD have done it and were much younger. I would tell them that we don't understand baby voices so if they want something they better repeat it in their normal voice, or we would flat out ignore them completely. 

Lots of irritating things over here for sure with a YSDalmost15 who is more like 12 emotionally.

I can only suggest...

Mouthswishing:  if your DH is ignoring it and SS does it walk away. Even if you are at the table, take your plate and everything and eat somewhere else. It's rude behavior, unhealthy and is a habit that should be stopped.  YSD was doing this wierd lip smack thing last year and I got on her about it (luckily DH let me) and eventually she stopped.

Babytalk:  Hated this, just hated it. I don't know why they do this! I have zero memory of my sister and I, or our friends doing this. It's attention seeking, that's it. I hard core ignore this. If I was addressed with baby talk I would not answer or I would make them ask correctly. I wasn't mean, just firm. If DH didn't stop it in general I'd leave the room. He didn't like it either and after a while when they got older started telling them to knock it off.

Food uneaten?  Do you buy the groceries?  No more eggs and no more bacon will be purchased. DH can buy it out of his own pocket. Yes, that means you may have to go without but think of the savings!  Buy breakfast stuff you like!

Prosecco or brandy.

Your DH needs to be the one to say "Remember last time you wanted a big breakfast and we ended up wasting eggs and bacon? I think you should just grab some cereal instead."

This is a DH problem. 

I need advice on this one too. Not to brag, but if there was a contest for world's most annoying stepkids I'm pretty sure I'd win, lol.

I have to pick and choose what is the most annoying, then talk with DH about how it triggers my anxiety. Like SS who doesn't talk but rather yells. His voice is like nails on a chalkboard. DH is finally getting it and corrects him. Most of the time. If DH doesn't see how something is annoying I will either leave the room or give him a look or clear my throat or something. Sometimes I just get really quiet. May be passive aggressive but it works usually. I have to be careful not to discipline SS's too much, middle especially because he is so overly sensitive and makes me feel like a bad guy. This is all more challenging when you are disengaged.

But there are so many things these kids do that annoy the crap out of me, I've learned it's best to try to make my appearance and polite small talk and then try to keep myself busy in other parts of the house or even leave to run errands. So basically avoiding them. Which is not fun to have to do in your own home.

It does help if DH is on board and supportive but he still doesn't get so many things his kids are constantly doing. Like humming for no reason. Playing on the computers in the main area and clicking clicking clicking, ugh! I may have that noise sensitivity too. I've actually told SS's at times to please stop or go somewhere else. I can't let those annoying behaviors go or just ignore them even though I wish I could. How can they not see how annoying they are being?