You are wrong.
For John to sell those 5 shares he has loaned out he will have to recall them from Bob.
If this is the sticking point, you're wrong. Both the original owner can put his 10 shares up for sale, and the new owner can sell his 5, at the same time. The shorter has to buy 5 shares on the open market to deliver to the original owner in time to settle his trade.
In the low volume case, if John uses a brokerage, no recall is forced, the shares come from the total pool.
Just like pulling $100 from your bank doesn't cause someone's mortgage to be foreclosed upon.
If all (or a large percentage) of the brokerage's holders sold (to other brokerages), it could cause a recall.
Yah this:
You're wrong too. John may not have loaned out his shares, the broker might have done it from a margin account. John has no clue that the shares were loaned, and doesn't have to do anything before selling them. That's why there are settlement periods, so that the broker can recall the shares (note: after they were sold) to effect settlement.