swg_tdr, as far as I understood it the last updates, dating from a few months ago I believe, dealt with overnight shorting. A lending plan had to accompany a short trade. Actually it had to since always but was often not done.
With intraday shorting we are talking 'intraday borrowing' (you may be short in the day but must be flat before close). It is different from lending as their is no documentation, nor any lending fee and it is at the discretion of the broker. Interactive Brokers for instance have lists of the total amount intraday borrowing that is allowed per type of share. Some brokers deal that in FIFO order, others give a maximum contingent to each customer, etc.
The broker indeed controls that through the sum of shares in his margin enabled portfolios...and yes they could easily cheat.
I agree it is hard to control UNLESS things go wrong and the customer couldn't cover before close. They will then probably do a margin call alike action in After hours to try to avoid undelivered or enter some last minute lending agreement.
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