Hello

I will try to guide. The Answer to most l of your questions are standard functions in Odoo.  I can recommend training courses at Odoo.som/slides

1. You do not need a purchase order to enter a vendor bill. Just go to the Accounting app and create a new bill.  Either upload a a new pdf / photo or just create. You do not need a product. You enter the expense account, prices, qty and VAT.  The other account account is coming from the vendor master for the partner ledger.

2. You don't enter in the Journal items. You enter new items in the Invoice lines.

3. Correct. You cannot delete a receipt that has been posted.  It is because when posted it create several posting in inventory, accounting and product costing.  These posting has to been reversed, so you need to use the Retunr function.  And after that, the transfers still must be there.  You keep the history of has been done.

4. Sorry, no standard functionality for this.  I can be done with creating a new reord rule. But it is quite a technical task to do.  You can read about some of it here: https://www.odoo.com/documentation/17.0/applications/general/users/access_rights.html

5. You return the product from the receipt. Press return and it create an outbound delivery back to vendor.  The outbound delivery got a reference both to the goods receipt and the purchase order. Warehoude manager should see the outbound delivery as ready to process. So you should expect that it is processed (?). To return the invoice you create a credit note from the invoice.  (Important to keep the history!) When you create a credit note from the invoice, you get a pop up with several options. If you need to inform the warehouse manager - send him a message in Odoo created from the invoice. (It is even possible to do this with automatic with actions.)

6. Be aware of the setting on the products in the purchase tab: Control policy. On ordered qty or received qty.  In order to modify qty on the goods rceipt, you need to Unlock the transfer.  I think you need the highest access right in the inventory app to have access to do it.  You can modify the ordered qty directly in the purchase order at any time.  But I recommend that you wait until you have received all of it. Else you can experience that it create new deliveries automatic.  You can cancel and delete deliveries that has not been processed.

Hope this helps you.