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Penetrating fluid?
Have you tried using 'penetrating' liquid to get into the tread and help to work it loose? One example is WD40 aerosol you get at the auto parts shop or you could ask a local plumber what they use. Another example is DWF ("de-watering fluid") aerosol lubricant and rust penetrant. This stuff "creeps" into fine cracks and may help to loosen the seized points.
If easy, remove the whole focuser from the scope, spray a small amount of WD40 (or other) spray ONLY into the affected area and leave that in for half an hour. NOTE: be very careful to only spray the few mm area that needs it; do not spill the oil on any other part of the focuser. If this proves too difficult you can also: spray some of the WD40 into a small bowl and then with a small paint brush pick this up and dab it at the entrance point of the screw.
Now do two things at once: Ask your wife or a friend to tap the base (area not visible when mounted) regularly with a small hammer "tic, tic, tic, tic, tic, tic, tic....." while at the same time you try to carefully undo the screw. Keep gently hammering while you work on the screw.
If it does not come free, try again in an hour's time. With the hammering the oil may have worked in a bit further, etc. Best of luck.
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