Hi crlhumans ,
Thanks for your post.
The only of your requirements that are partially met by ATUM (ATUM Multi-Inventory): You could add a product's inventory (virtual warehouse) per customer and use the Purchase Orders module to fulfil the inventories with the 'pre-ordered' stock.
However, the 'reserved area' that grants access to the customer to their virtual warehouses, would require custom personalization.
You could use the standard WC Orders and refunds, but you'd probably need customizations.
I hope this answers your questions.
Best Regards,
José Andrés