Well, got the nozzle cleared, finally!
It only took 2 days of fooling around, plus the use of a dremel tool with a cone-shaped grinding head, lots more torch heating, more use of my bicycle-spoke poker, partially dismantling the furnace, and a scraper of my own design.
Got that done too late in the day to try a new batch of lead in furnace--will do that tomorrow. I will flux faithfully! I will not heat the lead hotter than 650°. I will scoop off the dross regularly.
Learnt my lesson. Just hope the cleaned-out nozzle works!
Will keep y'all informed.
And a hearty Hi-Yo-Thankyou, to all who gave advice.