Welcome to our Forum. I will be glad to comment. Please accept my condolences for your loss. It is natural to worry that something you did in the past might have contributed to the loss of you wife's pregnancy. Syphilis however is almost certainly not the cause. In general syphilis is rare, even in commercial sex workers, condoms drastically reduce the risk of syphilis from genital sex, and syphilis from oral sex is very, very rare even among those who have syphilis. In addition, in answer to your specific questions:
1. I was very attentive to STD symptoms after exposures. I was focused on signs of urethritis, but I was looking down there a lot and would have noticed an obvious lesion. How often does syphilis produce symptoms that would go unnoticed even by someone on the lookout?
Syphilis typically causes a "primary lesion" at the site of inoculation. These lesions can go unnoticed if they are in a part of the body which is difficult to see like within he vagina or in the rectum but you would likely see a syphilis lesion on your penis if you acquired infection from the exposures you describe.
2. Is it likely my wife was screened for syphilis by her OBGYN, even though she is low risk on paper? We live in the US, have health insurance, and she received full prenatal care.
Yes, syphilis testing is recommended for ALL pregnant women early in pregnancy. It is almost automatic. If her doctor had not done this she/he would not be following widely accepted recommendations.
3. If the antibiotics cleared me up, am I likely reinfected by my wife? In other words, does a negative result mean she is likely negative?
A negative test result would reliably show that your wife was not infected. Antibiotics given for other reasons can sometimes inadvertently treat syphilis but this is unlikely in your case.
I am confident that your syphilis blood test will be negative. When it is it will completely rule out the possibility of syphilis in you.
I hope this comment is helpful. Please do not worry and again, accept my condolences for your loss. EWH