Heidi,
As I am looking at your plan, and keeping in mind the busy household that you have, I can see why you'd want to consider combining where possible. It just makes good sense!

I think the plan for your child in Beyond and the two in Preparing sounds like it will work well. My hesitation comes with the 7th turning 8th grader who was in CTC last year jumping all the way up to MTMM to join the 10th grader (who did WG last year as a 9th grader). Even if the 7th/8th grade child is able to make that kind of a leap for next year in skills (which is absolutely huge), the plan for the following year to jump that child up to the World History guide as a 9th grader would be something we would typically advise against. The only way we'd ever think a 9th grader could do the World History guide was if that particular child had come up through at least Rev2Rev, MTMM, and the World Geography Guide at the youngest age range of the guides and had really excelled in reading, writing, math, English, oral communication, and independent work habits.
Since the upper guides become quite independent, it is even more important to have a child well placed skill-wise. One other thing to consider is that combining at the upper levels isn't really as much of help as it is in the younger guides, since the work is geared to the student more than anything and there is less teaching. This is why I'd recommend taking a second look at your will be 8th grader and figuring out where he fits skill-wise with all thoughts of combining aside for now. The same is true for your will be 10th grader. Then, you will have a better idea if they could be combined, or should be combined, and if so where.
At the guides and age level that you are considering placement is the key!

If your oldest child did well with the World Geography guide, I would keep on going into the World History guide. The stair stepping of skills remains as important in high school as it was in the early years through HOD, and I just don't want your kiddos to miss the benefits of that by jumping backward and forward between guides that are meant to go in a different progression skill-wise.
If it was your first year with HOD last year, I am sure that it was an adjustment. Each year gets better as you know more what to expect, and so do your kiddos. Feel free to post your thoughts, because in the end you will know best as the parent. If you get a chance to post a bit more about where your two oldest fit individually on only the first page of the placement chart, then we could be better able to discuss some possible scenarios.
Blessings,
Carrie