Hi Tsu,
The information you are giving us isn’t quite clear to me yet. Do you mind if I clarify?
Assuming I have drawn the tree correctly, we can now add your husband’s adopted grandfather as the daughter of Marg EP, and the daughter of Norah we can now show as your husband’s match.
Now when you say “husband’s adopted grandfather” (let’s call him X), do you mean X was adopted by Marg EP, or that X was the son of Marg EP and he adopted your husband’s father? Whichever way, I think it means that Marg EP is not related biologically to your husband, is that correct?
If I have understood all this correctly (and it is quite complex, so I may not), then your husband must be related biologically to the H family, and adopted into the P family, which I think is what you are saying in your last paragraph.
It could be that someone in the H family had a child outside of marriage, and that child was adopted by the P family because they were connected by marriage. If this is so, it seems that your task is to estimate the number of generations back to the common ancestor between your husband and his match.
I don’t think I am telling you anything new, just working out for myself, but hopefully it may help confirm your thinking.
You don’t say what is the estimated match relationship but that will give you an idea of how far back you need to go - if an nth cousin than it will be n+1 generations back to the common ancestor. From the tree I have drawn, it seems the match is one generation further back than your husband. So, for example, if the match is 4th cousin, then it will be 5 generations back approximately - maybe 5 on the match's side and 6 on your husband's, or 5 on your husband's and 4 on the match's. That means that someone in your tree (Hugh H senior) may be the common ancestor, or maybe his parents. So that tells you where to start, by looking at children and grandchildren of the common ancestor.
I hope that helps.