1. With rare exceptions, late syphilis -- more than a year in duration -- is not sexually transmitted at all. For earlier syphilis, the risk can be very high for unprotected sex in the presence of genital, oral, or anal area symptoms like chancre (the open sore of primary syphilis) or seondary syphilis involving those body areas. It isn't possible to estimate a percentage chance of syphilis for any one exposure.  In general, anyone who has been sexually exposed to a partner with syphilis under a year in duration should be treated with penicillin before waiting for symptoms or a positive blood test.

2. No, condoms are not completely protective against syphilis -- or against any of the STIs transmitted primarily by skin-skin contact (including HPV and herpes). Comdoms are most reliable in preventing the infections transmitted through genital fluids (like gonorrhea, chlamydia and HIV).

3. For some kinds of blood test, almost everyone with syphilis continues to test positive for life even with successful treatment. However, the most commonly used type of initial blood tests (the "nontreponemal" tests including RPR) usually become negative, but not always. Some people continue to have positive blood tests after successful treatment. This also depends on the duration and stage of syphilis when treated:  the longer the duration or and later the stage of syphilis when treated, the more will continue to have positive blood tests for life. 

If you have had a possible exposure to syphilis and/or symptoms that concern you, let me know and I'll try to advise what you can expect about diagnosis and/or treatment.

HHH, MD