1) it isnt the recommended treatment. Personally, i dont think it will work. Note that bacterias may be resistant to some types of antibiotic. It is just playing with probabilities.
2) there isnt a text book answer. Everybody's reaction is different. But my opinion is that it shouldn't last for more than a few days.
3) i dont think that u should be assuming u had syphilis based on the symptoms. Please go get tested, good luck