I think this what I call a "jerk eating crisps" issue (someone eating crisps loudly would be annoying but I can ignore it as it's a small annoyance, if it's someone I hate or who has already pissed me off who is eating those same crisps loudly, it is the most grating noise in the world and I want to punch them but only because it is one more thing they're annoying me with)
The DCs coming and going is annoying, but since 3 out of the four have now technically moved out you've got used to them not being there, now they are around all the time it's even more annoying as they've taken over the space and are just always there when you've got used to the peace and quiet.
It's like when my Ex took DD on an extended trip last summer, they were away for about a month. I missed her terribly but after the first few days, I got used to her not being around - all her stuff was away in its place; whatever state I left the house in it was still in that state when I came home; I could eat what/when/how I wanted without having to think about what DD would want/like; I could go anywhere/do anything I wanted without having to consider DDs schedule or get someone to watch her etc. So when she came home, I was overjoyed to see her again but I did find it took a while to get back into a routine of factoring DD back into everyday life.
I definitely think you need to sit them all down and have an adult discussion about co-habiting together and treating the house like a home not a hotel. Then you can set some ground rules for visits etc. Middle DSD and your DS are home from Uni for the holidays, so I would assume they would always be around, but I would also have expected them to let you know they were coming home for X days/weeks of the holidays before they just turned up with a suitcase on your doorstep. Now they are there they should also be letting you know before they're inviting boy/girlfriends to sleep over or hang out in your house, equally it is courteous to let you know (whether by telling you, texting, leaving a note etc) if they will be out all night. As for PDAs, if it's a bit of cuddling on the sofa while watching tv or the odd peck on the lips, that's fine ... if it's playing tonsil hockey and wandering hands, that needs to be taken to a private space (aka a room without other people in). Also anyone using the room for it's legitimate function gets priority - so if Youngest DSD usually does her homework in the study but Middle DSD is in there playing tonsil hockey with her bf, MDSD has to vacate the room, even though she was there first, because a member of the household wants to use the room for its intended function while MDSD just needed an empty room iyswim?
Again, while they are home for the holidays I would assume to include them in any meal planning (so you cook enough to feed everyone but if they aren't there their portions can go in the fridge for them to have later or as leftovers) but I wouldn't be specifically catering for the veggie bf (unless you specifically invited him or your DSD told you ahead of time that they would be there). They should also be mindful of what they're eating - my DD is only 8 so she still asks before she gets herself a snack and I still cook all the meals, so I know exactly what is in my cupboards at any one time ... but when I lived with my Ex it was always annoying when I'd have ingredents in the fridge earmarked for dinner, go to cook and find my Ex had already helped himself to some/all of it earlier and now there wasn't enough left for the meal so I'd have to throw something else together.
Eldest DSD has moved out and has done for a while so there is no reason whatsoever for her to be bringing washing to your house unless her washing machine has broken or there's some other emergency. I wouldn't dream of turning up at my mum's house with a load of washing when I have a perfectly functional machine and washing line 5 minutes away! Middle DSD and DS are both technically living there so whilst they're on holiday I'd either have them put their washing in with the general family wash or I'd let them know when the washing machine was free (eg my mum's washing days have always been Saturday and Wednesday, whenever I visited from Uni I knew if it was one of those days I could throw my stuff into the wash with everyone elses, but if I wanted to do my own then it had to be another day/time as the households regular washing routine came first)
As for the washing up issue ... why is your DH washing 25 mugs in less than 24 hrs when there are only 6 people in the family?! That's 5 mugs per person! I don't even own that many mugs in the first place, but if my DD was using multiple cups/mugs for drinks and just leaving them on the side for me to wash up then we'd be having serious words about laziness and how she needs to reuse the same cup for multiple drinks, and if eg she wants water after a glass of orange juice, she needs to rinse her orange juice glass out and refill it, not get a new glass from the cupboard!