"All related archeaology within the archaeological horizon of a national monument monument should be considered of national importance"

Rocks the existing boat too much, and unnecessarily?

But the term "archaeological horizon" is inspired, and this part, on it's own, is great I think –
"even though as an individual structure it may be regarded as being of lesser important, its relationship to the major monument confers additional status"

I love that. Another way of saying it would be
"there should be a presumption that super henges should be treated as having "super settings" – both in significance and extent"
- which is plain common sense, since they were self evidently of great original importance.

I don't see why that assertion needs qualifying as "unless shown not to be". It should be a presumption.

The viewshed, I think, is a distraction that weakens it - it'll be bigger than the archaeological horizon (in this case anyway) so is an irrelevance. You might be able to see ten miles, but that won't wash.
The archaeological horizon thing, if adopted, would win the thing anyway.