Welcome to the forum.
Thanks for reading other discussions. You learned well: cunnilingus indeed is very low risk for any and all STDs, both for the oral and vaginal partner; and zero risk for some STDs. Directly to your questions:
1) Sore throat is a very rare STD symptom, and stiff neck for sure isn't due to STD. The large majority of oral gonorrhea or other STD cases are entirely without symptoms.
2) There would be no harm in having a throat culture (swab test) for gonorrhea, but I would not recommend it in this situation.
3) For sure zero risk for hep B, which is not known to be transmitted by cunnilingus. Syphilis could be, but rarely; and syphilis is so rare in hetersexuals these days (including sex workers) that there is no realistic chance your massage worker is infected.
If somehow I were in your situation, I would not be tested for anything and would continue unprotected sex with my wife without fear of transmitting any STD from this event.
I hope this has been helpful. Best wishes-- HHH, MD
Thank you so much for your time. This was very helpful and puts me at ease.