WooCommerce Google Merchant Center Fix

Need an experienced WooCommerce developer to repair Google for WooCommerce integration on a live store. The store is connected to a YouTube channel, but YouTube Shopping shows 0 products, so merchandise tagging is not working. Current issues include an inaccessible Merchant Center account, diagnostic errors pointing to an old Big Cartel account, 401 invalid credentials, failed sync jobs, paused scheduled jobs, a blank settings page, and a critical error after running the Merchant Center diagnostic. The account status shows connected despite errors. I need help identifying the root cause, recovering or reconnecting to the correct Merchant Center account, fixing the blank page and critical error, restoring product sync, and enabling YouTube Shopping tagging. This is a live store, so full backup first, no downtime, no duplicate products, and all products, URLs, variations, prices, inventory, orders, shipping, and other integrations must be preserved. Please document all changes. When applying, share a similar Merchant Center issue you fixed, what you would check first, and how you would avoid duplicates and downtime.