HI Andy,
What do you mean by "enhancements in APO to combine orders"? The (standard SAP) trim order in APO that can take the result from external trim - or have you created something on top?
To answer your questions (as much as I can :-)
You call it "configured KMAT" and I want to make sure I understand correctly: In this scenario it means real stock, which is carrying product specification, and resulted from excess production. If this was put to stock, it is visible to a gATP check of a subsequent sales order. A sequence of first ATP then CTP check in the sales order (this is standard function) could help you get rid of remainders.
Not sure why you want to use FERT, as you can stick to what you call configured KMAT (I'd call it a -partly- specified material variant). If you use batch management this stock can be found by ATP in inventory based on its specification. Of course, for ATP the stuff has to use an account assignment which is ideally "free available" and not a special stock. If you worked in special stock segments (because of MTO) think of changing to MTS segment, but still trigger individual requirements per sales order. If you afraid someone else will consume the material then use pegging.
I'm not saying this is an ideal solution but it helps. It comes with restrictions: The ATP can check on finished goods level only which may not be applicable as you still need to pack the goods you have found as remainder (and this is often a production step). This would mean that the goods to be found are intermediates which won't be visible to ATP. And the material availability for production orders is not as powerful as the one for sales orders.
To your second question: Why not use the function of lot-size calculation in PP/DS? SAP may not offer the right calculation method out of the box (depending on machine, run, product) , but I remember there were some user exits....
Regards,
Alfred