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How to record “short premium” in double-entry accounting? 
You don't. No one uses vanilla double entry accounting software for "Held-For-Trading Security".  Your broker or trading software is responsible for providing month-end statement of changes.  You use "Mark To Market" valuation at the end of each month.  For example, if your cash position is -$5000 and stock position is +$10000, all you do is write-up/down the account value to $5000.  There should be no sub-accounts for your "Investment" account in GNUCash.  So at the end of the month, there would be the following entries: