Slitherine

p132 states that if any part of a BG is in Severe Disorder that a CMT applies as if the whole BG is in Severe Disorder. Does the -1 modifier for a Cohesion test apply in the same manner? (ie only one stand in Severe Disorder). I can't find where this is explicitly stated.

Yes.
If part of a BG is disordered or severly disorded then the whole BG tests (CMT and CT) as disordered/serverly disordered.

For reference look at pages 26 as well as 132. Basically, you move and test by BG but you fight by base.