Welcome to the Forum.  I'll be glad to help.  Syphilis is a challenging disease to understand and is widely misunderstood.  Misinformation found on the internet just compounds this.  I have seen hundreds of cases of syphilis and do research on the disease.  Let me address your concerns in a point by point fashion.

1.  Your encounter was relatively low risk.  While U.S. syphilis rates are rising, over  nearly 2/3 of all syphilis occurs amongst men who have sex with other men and the disease is relatively rare in women.

2.  syphilis is almost never spread through receipt of unprotected oral sex and your use of a condom for the encounter is highly protective for syphilis acquisition.

3.  None of that however does not mean that uncommon things do not happen.  HOWEVER, your negative RPR results a year after the event is strong evidence that you did not get syphilis.  If your rash or swollen arm pit lymph nodes were due to syphilis, I can assure you that your RPR would have been positive. 

4.  On those rare occasions when untreated latent syphilis is associated with negative RPR or other tests for syphilis, the disease has been present for decades, not months or years.  Even then, for a blood test to become negative is vanishingly rare. 

5. That your rash is recurring in this was suggests some other, likely non-infectious dermatologic problem, not syphilis.

Bottom line, based on your multiple negative tests you can be entirely confident that you did not acquire syphilis (or HIV for that matter) from the February/March encounters you describe.  While I cannot tell you what is causing your current rash, I can assure you that it is not syphilis.  I hope this explanation is helpful.  EWH