In general, I agree strongly with this idea. In fact, I know it has been considered in the context of some other inventory-related changes.
But point #3 doesn’t make sense to me. I haven’t thought this through completely, I’ll admit, but it seems that one half of the transaction (debit or credit, depending on whether you’re writing off or writing on), will go automatically to Inventory on hand. The other half (credit or debit, as the case may be) will need to go to some income or expense account of your choosing. The idea of it ever going to Inventory - cost seems wrong, because money is not involved. Your inventory costs will be whatever they are, based on how much you’ve spent to acquire the items. You now just have more or fewer items to spread those costs over.
If items vanished, there is a debit to an expense account (such as Inventory spoilage) to offset a credit to the Inventory on hand asset account. I would want to be able to choose the most logical expense account. On the flip side, if you physically count more items than Manager says you have, a debit goes to Inventory on hand and you would need to choose an income account for the balancing credit. Maybe you would have an inventory adjustment income account; maybe you put it into other income.
Another possibility is to always post the balancing half of the transaction to an expense account, Inventory adjustments, on the theory that most inventory discrepancies are losses. The occasional write-on could go as a contra entry to that expense account. But you’d lose some visibility into the write-ons.