Western, if you don't want to go to the grave, don't go. You're allowed to change your mind. If you think rather than a time of quiet reflection with your memories of your dad it's more likely to be your mum mithering at you about this and that, then do something else. Go for a walk somewhere peaceful maybe, or do something that will (in a good way) remind you of your dad and honour his memory.
One of my dad's big loves was horseracing - I always feel a bit closer to him whenever I go. Made me think of him earlier when you said that your dad was only 49 when he died - it seems scarily young to me, as that's the age my dad was when I was born, and just starting on a whole new chapter of his life (I was his first child, and my mum, who he had only met a couple of years earlier, was his first proper relationship after nearly 30 years of playing the field...).
And re LM, depressed or not, he has behaved badly. And he's not even said one single sorry so far. He is not (yet) deserving of you sympathising with him. He could've said sorry, you're right, and then gone back to feeling woe is me, or whatever he's doing, but he hasn't even done that. Yet. I do stress the yet because I think (hope) he's not so foolish that he won't still redeem himself somehow.