Nick,
Q1) Is it possible to ATP in ECC and backorder processing in APO
No.
One would wonder why you would want this; since you can duplicate in APO any type of ATP that is available in ECC.
Q2) Do we have to have the ATP check integration model active for BOP to happen in APO.
Yes.
The above answers assume the standard SAP solutions. Obviously, if you have enough time and money and resources, anything is possible.
Best Regards,
DB49