Sue,
I was up late tonight pondering your post and thought I'd pop-in before heading off to bed. I'm glad that you updated us on your situation.
My advice to combine your older son with your two youngers for the history part of Beyond still stands. I think that it is a move worth pondering in light of what you shared about your busy family situation and your overwhelmed feeling you have when running multiple guides.
I do feel the need, however, to just clarify a few things.
I agree that it will work fine for your older child to combine with the youngers for the history portion of the Beyond guide and also for the Storytime part of the guide. However, you are right that the science is not enough for your older child in Beyond. In my thread before, this is why I had recommended keeping your older child going with the science, dictation, cursive, and upper level grammar from the Bigger Hearts guide. I know you were thinking to just wait on the science and do all three kiddos together in science next year when they get to Bigger. However, in order to keep your older child with your youngers through the years, you will need to use the science from a guide ahead every year. So, next year when your kiddos head as a group into Bigger, your older child will need to be doing the science from Preparing.
This fact will be true every year, as he will be beyond the target range for the science for each guide. History can be a bit more flexible in its age range, but science is not.
Another thing that I think is important to note is that I also advised a switch in the history, by downsizing your older child, in the hopes that it would provide you time to add in DITHR for him which I think is really needed at his age. Having the 3R's and science be on grade level for this child will be necessary in order for him to be getting what he needs.
Last, I suggested a downsize in order to give you more time within your day to teach your varying aged kiddos. However, if you end up with more planning to figure out how to raise the level of Beyond by adding in additional resources each day for your older child, then truly you would be better off just teaching Bigger to your child as written without all of the add-ons and additional planning to raise up a guide you feel is too young.
I agree that your son is outside the target age range in Beyond, but history and storytime can often stretch to a bit larger age range. The other skills cannot.
In order for this to work in the long haul, my thoughts are that you would need to have your older child join the youngers for the history read-aloud, the poetry, the Bible study box and music, the storytime, and possibly some of the hands-on activities (like the timeline). The rest of your older child's day would still come from Bigger Hearts (doing the dictation, science, upper level of grammar, DITHR, and cursive, along with a higher level of math). Without these additions from Bigger, the plan to combine the younger three will not work to prepare your older child for what is needed.
I do want the combining plan to work, but I don't want you to have to being searching, sifting, sorting, and planning to extend Beyond. Otherwise, truly your time would be better spent just keeping your son in Bigger. You'll know best what suits your family, but I couldn't go to sleep tonight without clarifying to both you and anyone else reading this thread what my intention was with my suggestion in this unique situation to place a 10 year old in just a few parts of Beyond.
Blessings,
Carrie
It was a great science day! 
But that makes sense. I figured since he would be on the high end of the extention level for Bigger next year, he wouldn't have to do Preparing Science. And, in Preparing at age 12, he would still be w/in the extention age as well. He is actually following the same progression as my ds13 who did Preparing at age 12. The only difference is we won't be skipping CtC for ds10.