Welcome to the forum. Thanks for your question.
You don't describe your exposure, so I cannot judge your risk from that standpoint.
Your dermatologist is correct: your local corticosteroid injections cannot have any effect on HIV testing. Potent, high dose immunosuppressive drugs, including ora high dose corticosteroids, in theory can delay conversion of HIV antibody tests to positive. (I stress "in theory". Even with these drugs, there are few if any actual known cases in which this happened.) But local triamcinolone (Kenalog) injections don't count. As implied by your doctor, this product is designed to produce high doses at the site of injection, but not enough of the drug is absorbed into the bloodstream to have any effect at all on HIV testing.
WBC counts normally vary widely. My own count in a recent medical evaluation was around 5,000, whereas it was 8,900 a few months ago. The differences mean nothing for either your immune system, or your general health, or for my own.
For those reasons, you can rely 100% on your negative HIV test results. Even the result at 9.5 weeks was 100% reliable; there was no reason for testing any later than that. For more information about HIV test timing, take a look at the thread linked below.
http://www.medhelp.org/posts/show/1704700
So all is well. I hope this has helped. Best wishes-- HHH, MD