OP, you've asked if anyone's got similar stories. I'll share mine, then give some advice.
I don't want to share too many details, but here's the short version. I lost beloved, dear, DM out of the blue 5 years ago. I grieved and continue to grieve every day. Many people will tell you that you never really get over a loss, you just learn to live with it. I think that's true. I think from everything I have read and all the expensive and painful therapy, that if you've had any kind of friction or issues in your relationship, it can be harder, not easier, because death is final. You have expressed several times that your DM is not here and you can't ask her why she's done what she's done. I think this just makes the inequity harder - you will never know if she was in her right mind (horrible, she preferred DB over you and acquiesced to his horrible will changing, hard to imagine someone at the end of life will of their own volition mostly disinherit... ), or if she wasn't (same thing, horrible DB conned her). You can't find out. What you can do is try to make yourself feel better by honoring your DF's wishes and screwing over your DB by not making it easy for him. You've said a lot talking to some stranger about it won't make you feel better, I understand that, I really do. But would it hurt to try one session with a grief counselor (wills and probate are after part of what hurts people later, and they are familiar with those things, hospice near you, even though DM didn't go that way, may still provide counselor for you...)
In my case I discovered something after my loss that was partly engineered by a family member and after a year of feeling "oh my god, my mum didn't tell me x" realized it wasn't even true. Was too deep in grief to get it. Other step-family member did something somewhat dodgy with inheritance, legally correct, morally questionable.
I am the kind of person who is driven mad by injustice. I can totally imagine being in your shoes and doing the same thing. Counselor steered me away and helped me see confronting either of these two would not help me with the loss. The loss is there. And also that I wasn't even in a good place psychologically to deal with the stress of legal or emotional issues, so I should let it go.
I let it go but I didn't and won't ever forget it or believe it was right. But what I didn't do is make myself even sadder and angrier than I was (NHS was a dreadful, dreadful, horrible shit-show let down). I didn't do a complaint against them either, although I was advised to do so, multiple times. Same reason - more pain to me. My mum loved me to death and would not have wanted me sad. I am so sorry you don't have that feeling about your mum, but you do about your dad. You want justice for him, but you know what he probably wants for you? Peace and happiness, that's what.
I am sorry you have had a hard time on the thread, if I was you, I would try and see the positive. Many people have been trying to convince you not to proceed. Not many have dug deep enough, or perhaps had the painful horrible experiences of losing two parents early and been screwed by a sibling, so can't see exactly WHY you need, for your own sanity, to not let this drop.
I also could not think straight or comprehend the way I usually do for a good year or two. So maybe you could consider that as well and like a PP said, take a friend to the solicitor (they are fine with it).
Please update what the solicitor says. There are many on here who want to help.
Hugs.