My DDs had been coming to me for (ever more extensive) respite for nearly 3 years, but when they were placed full time it was like being hit by a bus... it does get easier 
For now, reduce the workload as much as possible, and reduce the expectations you have of yourself..
Funnelling is important, but so is everyone's mental health. One or 2 people coming over regularly for coffee and a chat with you, while the children are also there, is not a disaster. Prep then to keep their engagement with the children to a minimum, without ignoring them.
Also, ask for specific, time limited support from your network. E.g. one cooked meal/load of laundry per week for the next 4 weeks, or whatever would help with the practical side.
On-line shopping for the bulk of what you need, only go out for things that are manageable and make a useful trip out as a family (nearby, when the shop is quiet, for a couple of small things).
What is the plan for the 5 year old going back to school? For DD2, who was 5 at placement, going back to school was a real benefit to her - she had time away from the intensity of her relationship with me, could settle into the routine and be distracted from her thoughts and feelings. Of course, that was from a basis of a pre-existing relationship, but other adoptive parents of older children have said that the normality of a school routine and mixing with other children really helped their child settle - it depends on the child. If they enjoy school and having other children around then an early-ish return is worth thinking about.
Would your 5 year old snuggle up and watch a DVD with you when the 1 year old naps? So they have your physical contact, but you can shut your eyes and take a mental break.
How about a timer, and saying 'Mummy needs to rest for 5 minutes, and then we will do XYZ' and using the timer so they can see how long before they have your attention again? You can gradually increase the time.