If you have been to three dentist who have all examined these bumps and all of them have given you the same dx, then I would feel confident in accepting that. Dentists ARE trained most rigouisly to identify cancerous lesions and if they have any doubt, they will send you to an Oral and Maxillofacial Surgeon who will take a biopsy for positive identification of the cells involved.
That your cousin died from oral cancer is fueling your fear and anxiety, as well it should. I am very sorry for your loss, let his untimely death be a huge wake up call for you.
As an ex-smoker myself, I don't say this lightly.......QUIT SMOKING!
If I can do it, ANYBODY can do it.
I wish you the best of luck. As smokers, that's what we count on, right?
Peace
Greenlydia