I think it's perfectly feasible, and I can totally 'get' your reasons for thinking it would suit. I don't have first hand experience of this age group (only HEd for a while later on) but my first thoughts are -
You wouldn't have to follow the NC, but if it was me I'd probably have a look at what the NC expects them to cover before the age you plan to start them, and check from time to time that you seem to be covering those sorts of things. But how you go about that would be totally up to you, it doesn't force you to do things in any particular order or any particular method. You'd probably find that you covered basics like literacy and numeracy anyway just because they'll 'happen' naturally.
Integrating into school - happy children will integrate, though they would have to get used to different rules than you have at home. But most children are used to that anyway, eg at Granny's house we do x, but at home we do Y. They might not know why, but they get used to the idea quickly enough.
CM - no problem, it would only depend on the CM having availability for that age group, and being happy about it. Legally, the fact that you HE doesn't mean you personally have to be with them during 'school hours', you can schedule your week/month/year how you like.
DH - tricky, but 'giving it a try' works well, as does explaining why you think it will help them.
As to it being selfish not to let them try school, you could equally say it would be selfish not to let them try HE! It's important that children get chances to be with other children, and to learn things like taking turns, coping with people they don't have much in common with, etc. But so long as they get that somewhere in their life, it doesn't have to be school.