I’m not sure who you mean they’re kept separate from either, Polkapink.
If you mean the junior school year 7 and 8s, then they are effectively in a separate school, with separate timetabling and curriculum, so there is no reason why the 2 groups should meet, have common sports teams etc. Doing so would surely defeat the point of having separate 11+ entry into the senior school? The boys who join the senior school at 11+ do so free of common entrance curriculum requirements - the boys in the junior school have largely joined at 7+ knowing they would do common entrance at 13, and do have to follow a separate curriculum.
King’s surely have to do their best by both sets of boys. But there is no reason why the senior school years 7 and 8 should follow the same, common entrance driven, curriculum as the junior school years 7 and 8. I’m sure king’s are perfectly aware of exactly where they want boys to be at year 9 when they come together, and will get them there in an engaging and varied manner, free of external constraints.
A number of highly academic schools, such as Hampton, are in a very similar situation, with significant intakes at both 11+ and 13+. Boys there will have come via Hampton years 7 and 8, but also from external preos at 13+. I don’t understand why the king’s scenario is different and troubling to you?
Your other objections seem to be around the added value relative to a grammar. Personally I would have ironed out my opinion on that decision before I made my child sit the kings 11+.