I don't want to encourage you to embark on this legal action because, as almost everyone is saying, there's a good chance it's an expensive hiding to nothing. But equally, nobody here can tell you for sure that it's not worth pursuing. You really need to rely on legal advice not ours, and, as I said upthread, I think you need more than one legal opinion in order to be reasonably sure you aren't just being ripped off by a solicitor who's happy for the hours to rack up fruitlessly.
Reading your updates and the various calls for you to get some counselling instead, I feel more and more that what would be most helpful for you is to do all the digging you can around what may actually have taken place, and really satisfy yourself you've got as many of the facts as you can, and then take stock of whether all of that adds up to something that's provable in court or not. Only then make a decision about whether to plough on with legal action. I think that process would be therapeutic in itself.
In your shoes, I would go through the medical records carefully, with a medical professional if you have a suitable friend who can decode and shed light, and see if there are any alarm bells. If she had a GP who saw her regularly and knew her, I might arrange to talk to them about your concerns. Who witnessed the new will? If possible, I would talk to them about the circumstances and any concerns. Are there neighbours who might have seen the kind of treatment she was receiving? Etc. Only if you turn up something that's worthy of a court's time should you proceed with the legal action. But I think if you feel you've done everything you can to get to the truth, you will feel better able to move on if legal action isn't possible.
I am so sorry for your situation and wish you luck.