1. Mutual masturbation does not put you at any risk of HIV
2 As long as the condom remains intact, it is safe, regardless of its expiry date. There is nothing like cracks or pinhole in the condom
3 Syphilis is 100% curable STD therefore if she had it in the past, it must have been cured. Condoms provide protection against Syphilis too anyway
4. Even if the condom was reused, it still is not an HIV threat if it is intact. Though very remote possibility, still we assume the reuse theory, it is not a risk for you if you performed oral with that
5. As long as the condom covers head of penis (urethra and foreskin if you have), you are safe, no matter how limp your penis was.
It looks that overall risk situation didn't exist. Further if your antibody test was done 3 months from your last exposure, it was conclusive and rules out your risk completely
Your test is conclusive because nothing you posted is a risk for HIV. Only unprotected anal or vaginal sex and sharing IV drug needles with infected users are a risk for HIV in an adult.
As for how to get rid of your anxiety, you should seek therapy. That's the only way you'll get rid of your anxiety.