Hi Bramblina
Although I have not experienced exactly what you are going through, some of it I do (eg- DP away etc).
I think with your eldest there are a couple of things going on - the onset of puberty (which has psychological changes in brain chemistry/behaviour as well as the physical changes), and also the transition to secondary school. Perhaps the reason he has coped with the change so well, is because of a new coping mechanism - to be sarcastic, and back-chatting - in a way that gets approval from his peers?
With three kids and a husband away a lot of the time - you must be pretty exhausted - and it must be really hard to be consistent in enforcing your rules and punishments, but it is great that grounding your son worked. I would imagine it would be a good idea to use that again? Also, it is probably a good idea to chat to your son in a more laid back time - not when he has done something wrong - and find out how things really are for him on the inside, and explain that you are his mother, and although you want to give him the best of everything, it is important for the well-being of everyone in the family that he treats everyone with respect, and it is with heavy heart that you will need to keep grounding him until he gets out of some of these bad new habits.
I think the daughter with head in the clouds - maybe find out where her head is? Ask her what she likes thinking about or imagining or what is on her mind. Speak to her about the fact that she often doesn't notice dropping her clothes where she is, etc - ask her if she is aware of not noticing. Ask her if she has any ideas on what you can do to bring her attention into the room...
Basically I think the place to start is communication, support and getting to know your kids as people again (they are always growing and changing aren't they?), and perhaps try to see them less as just the offspring you need to feed, clothe, water and shepherd all by yourself.
For me, I have found an amazing turn around by making tea with biscuits/cakes/sandwiches immediately after getting back from school and just talking the day through in a window of 'screen-free' time. It is astonishing to think now about how much of an insight into their world I must have been missing before I started doing it... This routine of chatting things through - (how our days went, what we plan to do for the weekend, what is bothering us, what we are excited about, etc)- seems to make everything else run so much more smoothly.. I wonder if is in part because they are getting positive attention ages before they start doing attention seeking things, so they feel happier? I recommend it though - even if it seems you have too much on your plate to do it - honestly - it seems to actually create time..