Well, I can add some pretty up-close and personal experience about INTPs and suffering in the marriage due to an ill spouse, and what it's like to be a child of that.
My father is an INTP and last year my (SJ) step-mother was finally diagnosed with multiple sclerosis after years of strange bouts of illness. The toll on her health has taken a rapid downturn in the past few months. When I visited my father last fall I knew he was anxious by his incessant, factual rambling. He kept telling me "I've thought things through a lot, we're gonna be fine, she's just gonna work less, we're gonna save more, we're gonna opt for cheaper cars, we're gonna find a cheaper house, we're gonna sell the extra property, we're gonna make sure you kids are set up for whatever you need, I've read about some alternative treatments that we're going to contact the insurance about" etc. I know my Dad. I know he's good at thinking about the future and trying to cover all the angles. I know he's given the probable outcome of all of this a LOT of thought. It is touching to see how he has gone through great pains to fully educate himself on all of the implications of her illness in an effort to be helpful and informed, which is how he shows his strength and support. He has spent hours poring over their financial investments and savings etc. to see what their resources are and the best way to use them.
But I also know that my Dad feels it is his job to be the tough, responsible one at all times... an oldest child like me with lots of responsibility thrust onto him at an early age. He bottles up his feelings not just as a behavioral preference, but because he does not have any close friendships where he can vent his feelings of sadness, frustration, and anxiety that I know he is experiencing. I think with his wife and kids especially, he has avoided appearing frightened or sad because he wants to be a stable, hopeful, and positive presence for us and I appreciate what that stoicism says about his love and care for us even if we don't require it (but it's obvious that he requires it more for himself). I'm a good reader of my Dad, and I too have worried that he is bottling up more than he can handle on his own. It would not be the first time he has done this. I know it is in his nature to analyze the freak out of situations so that he can be mentally prepared for all of the variables, but I have taken the time to stop him on two occasions and said "I know you've thought this all through Dad and I know you're taking good care of her, but what about you? Are you okay Dad?" And this is where I see the hesitation, and the facial muscles at work, and he says matter of factly, "I am scared." ... *silence* And this is where lots of hugs and reassurances came in, both verbal (but often) silent: a squeeze of the hand, a surprise coffee from Starbucks (his favorite), or an offer to help him and my stepmom out in a tangible way (want me to make dinner? walk the dogs? help research alternative treatments etc.?). Just talking about his feelings is not something he is often interested in I have learned. My Dad doesn't have to elaborate on his feelings though if he doesn't want to, I get it. But it is still important to me that he knows I care, I am here, and he can lean on me and my adult brothers if he needs to because that is what family is for: he doesn't have to be SuperDad all the time. I know it's just his role in my life that prevents him from having that much-needed ventilation, so I was relieved when we visited my grandmother (his Mom) and she asked if he was okay after he got done telling her all of the factual details and plans etc. that he had just told me, but then he just broke down sobbing, and we group-hugged for a long time and didn't say anything except that we loved each other. My father is not a cryer, and seeing him cry like that felt like having my heart ripped out of my chest. I know he is in pain and I think his venting made us all feel relieved (my grandmother and I because he's shown he can be vulnerable with us and my Dad because he got to see that we can handle it).
I guess my final thought is that if your father says he is fine, you will have to give him the benefit of the doubt. I'm sure the rapid change of lifestyle and partner must be having a tumultuous emotional impact on you (it would on me). All you can really do is let him know that you care, that you are concerned, that he is not alone, and that you support his choices if it means he will be happy even if you don't understand them or wouldn't choose them for yourself. Maybe saying that will allow you both to reassure each other in a meaningful way. IME, just proving yourself as a reliable source of support, a patient listener, and reaching out across the aisle at those moments that you sense they want to but are hesitant to, is usually enough for INTPs and most other people.
Saturned, I am so sorry for your loss and I dread the day that I may have to make a post similar to yours.