> 1) a 100 ohm resistor in series with the speaker / headphone socket,
> always in circuit regardless of whether the headphones are plugged in,
> for short circuit protection, and

h/p will be only a few mW so you must have that resistor unloess you
like your sound unusually loud for a brief time

> 2) an audio transformer to drive the headphones; this comes into
> circuit only when the speakers are cut out, and would give complete
> isolation to the headphones?

Shouldn't be necessary. A transformer is normally used for
impedance matching - what you are looking for is voltage
matching (or rather voltage dropping, therefore a resistor)

> The only issue I foresee is that I still would short the circuit at the
> socket while inserting the headphones, but now it would be thru'
> the 100 ohm reistor. Would this work? And, would it be safe?

The socket shouldn't short. Not in the way you think perhaps. The
tip and ring contacts on the plug shouldn't be able to be shorted to
the barrel of the plug via the socket contacts, nor should the tip
or ring be able to short out the contacts inside the socket. However,
if you think there is shorting then the 100R will really help

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