In the nicest possible way OP, I think you are deep in grief and there is a lot invested in this emotionally for you which is making it personal and upsetting and you are lashing out at anyone who disagrees.
I had a similar situation with a much loved family member - in all honesty I don't 'feel' that how it turned out would have been what she might have gone for if she'd had all the facts and a cogent mind- BUT a large part of that boils down to not wanting to feel she may have cared more for others, wasnt bothered about me, maybe I didnt know her etc. All of which, years later with a clearer mind, way well be true (which is fine) bit equally I do not think it was about that. I wouldnt have contested the will though in a million years, she made it and whether that pleases me or not she had options and chose the ones that made her feel most comfortable in that moment, as a person who was unwell. It is a difficult time. What is 'fair' is very subjective and is up to the person making the will to decide- your opinion on it doesn't play a part.
I would seriously recommend talking to a grief counsellor before you spend too much money on a solicitor- you are very unlikely to get anywhere and seem not to be considering how far in debt YOU will potentially end up just to spite someone else because they have something you want- both emotionally and re:the money.
You will need to prove what you allege beyond reasonable doubt and you most likely cannot. The fact you can provide a logical argument that makes sense to you is not the point. You need to prove it DID happen, not that it could. Talk to the solicitor by all means BUT be very careful to provide facts only and not your opinion. The solicitor will go by what you tell them and you'll have wasted hundreds by the time it is clear there is little evidence for it in the legal sense.
I'm sure your relative would not want you to be upset in this way OP- arrange grief counselling for your own sake.