Hi,
1. You can go to QA12 (Edit UD) and post the remaining quantity whenever you need. The inspection lot status will remain SPRQ (Stock posting required) till then. Once all the quantities are released, then status will change to SPCO (Stock posting completed).
2. This requires customization. There's no standard control on inventory level and UD. Please check with ABAP team to place custom controls at UD level.
Thanks
Prem