ES,
Good post!
Yes, the best option is to assign the checking group (or availability check) = KP, for materials for those specific plants for which business does not want availability check to be carried out.
I think, even with CP as the schedule line category, in the sales order, the availability check will overwrite and no check will take place. Please test this.
If it is this way, then no need to create a new schedule line category.
tw