While I am glad the "truth" eventually came out for the first story, it's very noteworthy that the OP consistently let his mother "get ahead" of the truth at every turn. He squandered his chance to confront his bio father, squandered his chance to be the one to break the news to his grandmother. And ultimately that's reflected by everyone in the family being okay with it all, somehow, and him never being able to tell what is a truth or a lie. He gave his mom, and potentially also his dad, time to get their story straight, and now the only story he'll ever know bears all the telltale marks of a narcissist's on-the-spot lie.
I knew i watched too much of those Reddit videos when after op clarified that grandad is their mother's step dad and i immediately got where this is going