You did not have a risk for HIV from mutual mastubation ...You do not need testing!
Even with broken skin...HIV is not tranmitted from mutual mastubation even with nicks, cuts and broken skin. The virus is inactive outside the body! Transmission happens inside the body as in unprotected vaginal/anal sex, sharing needles for drug use and mother to child!
at the testing centre they said that there was "some" risk , and websites say so too.... but is my negative test at 14 days antigen reliable?
You do not have a concern for HIV. You did not need to test in the 1st place and seeing that you did ... you can trust your negative result and not worry about mutual mastubation in the future!
At no time were you ever at risk of contracting HIV from masturbation.