Regarding first query, Components will be copied into Subcontracting PO provided BOM is maintained for the Parent Material codes. Else you will have to manually maintain the components while creating the PO.
Which procedure is followed by your client?
There is no such control which will restrict PO creation where Component is not available in SAP system.
You may try writing the custom code. But, i personally believe that writing custom code will worsen your system performance..
Regarding Second query, If the Subcontractor failed to send the Material with in 180 days, you will have to reverse the Cenvat credit taken while procuring the Raw Material. This is mentioned several times in the forum. Search in forum and you will get no. of threads.