Listening to your story, it's hard to imagine that your W's (seemingly untreatable) headaches aren't connected to her emotional life - particularly given that they started around the time of her parents' D, and seem to continue to effect so strongly the contact that she has with you, and her ability to work.
You said FIL wants to blame you for the headaches (due to concussion), and only wants to seek medical solutions to the problem. I guess that suits him in two ways: it takes any possible responsibility for her condition off him, and it prevents the situation being addressed on an emotional level.
It sounds like FIL has powerful needs to cut off emotional scenarios in his own life: for example refusing MIL's wish to see her at the end of her life, refusing to attend SIL's wedding etc. Sounds like that world of emotion is too dangerous for him and he has massive defences up against it - presumably developed as a kid in response to his own alcoholic tyrant father. His immediate remarriage after the D also sounds like it might have been blocking out emotions - not taking responsibility for his part in the failed M, but instead (I speculate) blaming MIL. I wonder if he got the kids to buy that line - i.e. bringing about your W's belief that MIL was 'the crazy mom who had abandoned her kids'.
My guess is that your W must have deeply divided feelings about FIL, not least because after the kids sided with him (after the D), he then betrayed your W's trust by remarrying quickly and breaking his promise to her. No wonder that's a sore spot for her. It shows FIL getting it wrong, and she may need to believe that he has got things right.
The reason I mention all this is because my sense (and this may be completely wrong) is that it might be worth thinking carefully about what you confronting FIL might mean to your W.
Here I respectfully disagree with Dday Of course, Dday may be right, but it's possible, I think, that a big part of your W's identity (her big 'mistakes' in life) are tied up with FIL: in particular, following his take on his own marriage and the 'crazy mom' thesis, and also - like FIL - following his example of failing (or refusing to) grieve your MIL's death. This is why I say she may need to believe that FIL has got things right: imagine what it would take to believe that FIL's take on the world is wrong. It might mean admitting that he was wrong about MIL, but she has passed away, so that damage can never be repaired. In other words, if FIL is 'wrong', then both of those matters (crazy mom/not grieving) become a kind of a betrayal of your W's mother, so they are likely to be extremely sensitive.
For these reasons, I wouldn't underestimate the power of those points of connection between W and FIL. I think that power is evident in the way Your W seems to be caught up in his vortex - sometimes breaking free from it, and things being better with you, and then being sucked back in again.
My concern is this: if you dramatically challenge him, it's possible that you'll also be striking at a very confused set of issues within her, and she may react by siding with him more strongly in order to face those issues down.
I think it's significant, and as Dday said, curious, that your W opposes you standing up to FIL.
Let's say for a minute that the idea above is right, and that a 'hard' attack on FIL (by you) would also indirectly be an attack on unresolved issues within your W. Is there an alternative? I think there is, because alongside the unhelpful ways in which your W has followed FIL, she's also rebelled against him too, and she did that through a 'soft' attack, through love, by marrying you.
Isn't it interesting that he really wants a hard-headed businessmen for son-in-laws, but as Dday mentioned earlier, "both W and her sister in their own way rebel against the F by chosing those completely different from him": you with the ministry and music (and your ongoing, loving DBing), BIL with the counselling.
In the light of the sisters choices', I wonder if there's another reason why she doesn't want you to attack FIL - isn't hard attack FIL's own personal way of doing things? And by attacking back in that manner, wouldn't that make you like him?
What's your feeling about this? Is it possible that she wants you to *not* be like him? After all, it sounds like you weren't like him when you and W were first together. You mentioned that at an early stage, everything in your marriage became about proving FIL wrong, but that sort of sounds like you had to adopt his own work ethic--and to become like him--in order to prove that you weren't like him. Ironic if true.
I suppose my question is, what is your sense of how your W feels about the times when you've acted more like your FIL in the past, and how does she respond when you are (in attitude) more like you were when you first met (i.e. big contrast to FIL)?
I thought it was encouraging to hear that BIL had asked your W about how she has dealt with the grief over your MIL's death, and particularly that she repeated this coversation back to you. It's a kind of acknowledgment on her part, even if the subject is still unaddressed. It also sounds like BIL has clear insight into the dynamic. It's a happy coincidence that your MIL described your FIL as a dry alcoholic, and that your BIL is an addiction recovery counsellor. It's also good to hear about how well SIL dealt with MIL's final illness. Are you in contact with them? They sound like a real positive in this situation, and may have much insight to offer, and, a relationship with your W through which that may bear fruit.
Of course, Dday may be right, but it's possible, I think, that a big part of your W's identity (her big 'mistakes' in life) are tied up with FIL: in particular, following his take on his own marriage and the 'crazy mom' thesis, and also - like FIL - following his example of failing (or refusing to) grieve your MIL's death. 