It's understandable you are upset to have had it implied to you that your past is not as straightforward as you thought it was.
Do please try and keep in mind that it is extremely rare for the fault to be all on one side in any situation. Of course this can occasionally happen - sometimes someone is the victim of an abuser for example - but often in relationship breakdowns nobody is completely the good person or the bad person - it's more complex than that.
Your mom may have made mistakes, and she may have exaggerated your dad's faults to you. Equally she may have believed for whatever reason that she was doing you a favour keeping you away from him (rightly or wrongly).
Your dad may indeed have loved you and wanted to see you all along. That doesn't mean he was perfect or that he didn't make mistakes himself.
Surely the ideal goal for you would be to have the best relationship you can with both parents, while accepting that both are imperfect and may have made mistakes.
I would speak to both of them (separately), and your gran, as calmly as you can. Perhaps write a list of the questions you would like answers too. But you also need to accept that you may never get those answers, or certainly not as fully as you might like. Mom, dad and gran all probably believe that on the whole they acted for the best - people generally do believe this. They may, with hindsight, feel able to admit some fault in their actions. Or they may not.
Ultimately, if you feel that they love you then it's probably worth working towards forgiveness and building a relationship.
Have I always done the right thing with regards to my daughter? 100% not. Do I love her so much I would lay down my life for her? 100% yes.