Hi Sheril,
Don't lose any sleep at night. I know that's easier said than done, but try not to let it get on top of you.
I'm not sure that I'm the most knowledgeable person on domicile/joint sponsorship. My circumstances for the affidavit of support were: me USC, unemployed housewife of seven years. DH British/wage earner. I was the sponsor, and we used our joint assets so we didn't require a joint sponsor or to domicile.
You seem to have a good understanding of what they want from the i 864: a salary of $xx or assetts of $125% above the poverty line, for the number of people in your household. If I remember correctly, I
think, but I'm not 100%, that the combined salaries of the sponsor and joint sponsor need to meet the required income. i.e. If the sponsor's is not enough, the joint sponsor will make up the difference.
They want to make sure that you will be able to support yourselves.
It would seem to me that if you already have a relative (SIL) who can sponsor you and already did so once before, that it would make sense to for them to joint sponsor again. Having extra support would be in your favour.
As for your question about whether your husband should go to US and seek US based employment vs. staying in UK where he is currently employed, I can't answer that. Perhaps somebody who was in that specific situation can shed some light?
I'm sorry I don't have more concrete answers to your circumstances. You are right that the process and paperwork is formatted for US based filing. DCF does understand that. Still, I know that you want to make sure everything is order, and how confusing the information is.
I'm sure somebody who repatriated in similar circumstances to yourself will reply and give you some advice. In the meantime, try not to worry too much.