Thanks,
Thoughts are moving away from a plug adapter to a cable based solution.
Thus what is required is a standard (larger) square optical plug for the tv output and either a mini square Toslink plug or 3.5mm audio jack for input to the Bluetooth transmitter - as you indicate. This assumes that an audio jack will work with an optical plug and does not require any 'splitting' (not understood).
An alternative may be a cable with 3.5mm at both ends which then assumes that the tv optical output is compatible; using the tv audio output (3.5mm) not an option because it inhibits the tv sound.