Peter,
Not sure I understand your comment on "insets." Can you explain?
I have used vertically oriented grain on some racing planes with good results. It is assembled with 30 minute epoxy, and the plywood doublers are used to unify the structure. I have also used a maple block with the grain parallel to better ensure an optimum glue joint. Let me see if I can find a picture or two.
The advantage of orienting the balsa one way versus another is due to material anisotropy--the strength of balsa is very different between along grain and across grain. Orienting it between bearers with the grain at 90 degrees it is intended to increase the crushing strength between the two bearers. Say, if you buried the nose of the plane in the ground coming in at a 30 degree angle. The bottom motor mount would like to displace upwards, which would require the front of the cross piece to crush; or, the bond joint at the back to separate. Note that with a properly glued and unified front end, the ply doublers would have to shear and/or buckle for the mount to move. That's what you want--all the pieces contributing to taking up loads.
Remember that combining all these wood types and in different orientations is the very definition of composite construction. When you consider a certain design load--like a nose-in crash--you are imagining where the load gets applied, how much it will be, and so on. Note that the stiffest piece in the composite arrangement will take up load first and may fail despite all the "heavy reinforcement" buried somewhere else. You need to think of "maximum fiber" which is the engineering concept for the outermost material surface from the neutral bending axis. So in this case, the hardwoods and the plywoods are much stiffer than the balsa, whether the balsa is measured with- or cross-grain. With plywood doublers on the outside, then what the balsa core is mostly doing is preventing buckling of the plywood and carrying internal shear from bending loads.
On another plane, I used another piece of hardwood where this crossgrain balsa separator might go. also worked, and the weight up front didn't hurt anything. And, it provided a place to install bolt-thru landing gear. More functionality from the same increment of weight.
The problem usually with "overbuilding" is that we stiffen one area which then focuses loads/bending in another area and the end result is less durable than it was. Classic trial by error "whack-a-mole design...."
Dave