Consider this strictly a discussion.
Yes, I am syncing total Amazon fba stocks in my WC. The amount reflected is a total of all 7 warehouses. Obviously the plugin doing so is not ATUM compatible.
An API for this is a brilliant idea. I have been juggling with Amazon api for about 5 years and I may be able to guide you in the correct direction.
Amazon does provide a report, which gives you a breakdown of all your inventories in all there different warehouses accross country. The primary relation with WC and Amazon integration is the SKUs on both these marketplaces should be a match.
So on one hand you have Amazon report, showing product SKU (matching with WC sku) and its mutiple inventories. If ATUM could create multiple inventories after processing this report for the products with same SKU and map it using SKU's fnSKU (fnSKU is Amazon's sku for a batch of inventory of same SKU - like a sub-SKU) to Supplier's SKU, it could in some way work and sync harmoniously.