An additional assumption I didn’t realize I was making was I was thinking the XZ parts would clamp the tubes in such a way that the distance between them would vary too. So the outer pieces accommodating variable width is handling the tube size and also the tube spacing. But the concept is not tied to a particular sequence of certain dimensions being independent and other dimensions being dependent, so it’s whatever works.
Also the 608 bearings are 7mm wide with an 8mm inside diameter, but some bearings are slightly beveled on the edges so the usable width might be a slight bit less. I think we are in agreement on the concept of using the entire bearing width even though the numbers are not the same.
The lateral force of the screws holding tension through the plate is a valid concern. The plate is being asked to do more duty than before so it is a potential risk of discovering a new weakness.
Another possibility which is perhaps interesting is that the bearing supports do not necessarily need to be coplanar. I’m not sure if there is any advantage to offsetting them, except that it might help make room for beefier parts and more distance between anchor points.