Eric,
I will confine my remarks to 'standard ATP functionality in ECC'.
You can set up an individual Finished Goods material so that Sales orders placed against that material will perform an availability check against unconsumed forecast. The forecast can be against individual FGs materials, or it can be against a planning material, which represents a collection of FGs materials. In both of these cases, if there is no forecast, or the amount of forecast is too small, then the order will either remain unconfirmed or only partially confirmed. This seems to adhere to one of your business requirements. Bear in mind, though, that this availability check ignores stock and any other supply element. You could have millions of pieces on the shelf, but if there is no unconsumed forecast, the order will not confirm. As more sales orders are added, they consume the forecast, and thus reduce the remaining amount of forecast available for ATP checking.
For more info about these scenarios, see
Planning w/o Final Assembly and w/oMTO (52) - Demand Management (PP-MP-DEM) - SAP Library
Planning w/ a Planning Material and w/oMTO (63) - Demand Management (PP-MP-DEM) - SAP Library
It is also possible to set up an individual Finished Goods material so that Sales orders placed against that material will perform an Assemble-To-Order availability check. In this case, ATO can be configured to confirm sales orders based upon the availability of HALBs. Unfortunately, ATO in general is considered to be a subset of MTO, so all the MTO restrictions apply.
For more info about ATO see:
Assemble-to-order - Demand Management (PP-MP-DEM) - SAP Library
There might also be some opportunity using product allocation instead of forecast. In this case, you can enter product allocations instead of planned independent requirements. During Sales order entry, if the system will pass a standard ATP check against Finished goods, and the sales quantity is also equal to or less than the product allocation quantity, then the Sales order can be confirmed. For more info about Product allocation see:
Product Allocation - Availability Check and Requirements in Sales and Distribution Pr - SAP Library
Except for these partial solutions, I am not aware of any standard ATP check in ECC that will meet all of your requirements.
Best Regards,
DB49