Yes… But if NAS, Linux1 and Linux2 are already sharing A, they also have B and C which are subfolders. So no need to explicitly share B and C once more?
I don’t see why doing so would break in the way you describe, but it’s going to be very confusing for syncthing anyhow. For example a change in B would also be a change in A, so it would try to sync the same change twice…