Welcome to the forum. Thanks for your question.
Sometimes the title of a question has most or all the information for an accurate reply. "Likelihood of STDs from frottage"? Before reading anything else, I would say zero or close to it. Any symptoms probably are not due to any STD from such an exposure.
Now I have read the rest. A more nuanced response is that there conceivably could be a chance of acquiring one of the STDs transmitted primarily by skin-to-skin contact, i.e. herpes, HPV, or syphilis. But in nearly 4 decades in the STD business, I have never seen a case of any of these in someone who didn't have a riskier exposure, i.e. penetrating intercourse. So for practical purposes, you can consider this exposure to have been risk-free.
I don't recommend HSV testing even after unprotected sex. It's not a wise thing for you to do; the tests just aren't good enough, and a positive result would not be reliable and would not indicate infection from this event. And I certainly see no need for testing for anything else either.
Myt advice is that you take pride in having safe sex (no penetration), secure in the knowledge you aren't at risk. If you have a regular partner, you can safely continue unprotected sex without risk of transmitting anything.
Regards-- HHH, MD
You're welcome. Glad to have helped.
Thank you so much for the quick reply. Your expert answers make me feel much better and help me to really relax about everything. Even though it's considered safe sex, I definitely won't be doing anything like that again mainly so I don't worry so much about it. Once again thank you so much.