Supporting Documents and Purchase/Export Quantities for Issuing a Local L/C (Multi-stage Assembly Export)

We are a manufacturing company. Our logistics flow is as follows: Overseas production subsidiary (semi-finished goods) → Domestic partner (sales subsidiary) → Our company (purchase and finished goods assembly) → Export to overseas customers. Our company receives semi-finished goods that our domestic partner has imported from overseas. The partner has requested the issuance of a Local L/C (Purchase Confirmation) so that they can issue a zero-rated tax invoice after we pay them the goods' value in foreign currency. We plan to assemble these into finished goods and export them later. Question 1) When issuing a Local L/C, export-related documents must be registered as supporting documents. Would a supply contract between the customer and our company be valid? (The unit price of the finished goods is specified in the contract, but the supply quantity is not designated.) Question 2) If we import 10 units from our partner, must the export also be exactly 10 units based on the finished goods? Is it problematic if there is a discrepancy between the import and export quantities due to defects or other reasons?