OK, prepare for a rather long one...hope you are feeling a bit better, first of all. And in terms of the blip: if one of your kids was ill and cranky and being driven mad by itching all the time, you would hardly expect them to be firing on all cylinders and reacting with sweetness and light, would you? So be kind to yourself and don't overplay what happens while you are trying to recover.
I was thinking that zoning in on good behaviour might help things along. Explain that you want them to notice when you know they have behaved well. An amazing person I know, with 3 DCs, aged 9, 6 and 2, described the 'thumbs up' system to me and said it worked very well. Whenever you or DH catch one of the DCs doing something kind to the others and to you, and/or do things well that you have asked them to do, you can give them a thumbs up. This could even be a printed thumbprint in paint on a small square of paper - could your 2 younger ones make you a load of these? 30 per DC probably. You put them in a jar (high on a shelf, or in your room...) with their initials written on the paper square when awarded.
Now, I as an only child would have simply thought that getting the most thumbs ups was the name of the game, but apparently the element of chance is quite important. At the end of a month, mix them all up well and put them in a bag, and you pick out a winning thumbs up. The more you have in there, of course, the more likely it is to win. That DC receives a reward, could be something little like a magazine but personally I think things that would be more motivating are: they get to choose what you all have for dinner/ for pudding one day; they get to hire a film of their choice and have a film night with popcorn etc; they go to a bouncy castle; they get a £5 argos gift card or something they can make choices with; they have a mini cereal selection with weird & wonderful cereals for a week. Then you can make the new thumbs up papers again (got to be good for 30 mins, right?)
2nd thought: Back this up in the day with phrases like: "You did exactly what I asked you to with no fuss, that's great. Thank you." A friend of ours had asked her pre-schooler to go and stand near the ticket gates at a station while she took a big case through the other one, but then was talking to us for a little bit longer, and as it was busy, asked her DD to come back to her. She said that to her, following up with: "...even though I changed my mind and wasn't ready to come straight away." I thought that was effective as her instructions were potentially contradictory.
3rd thought: in 1:1 time with the DCs when you are putting them to bed, maybe say something like: "I really enjoyed playing with the silly string today with you. I didn't enjoy having to tell you off about X because I felt as if you were not listening to me/ being a bit rude, but on the whole I enjoyed what we did". Be sincere about it - only use when there ARE positives to balance up the negative, that you then refer to but play down a little rather than dredge it up again.
There was something else but have forgotten it for the moment!