That sounds like the classic problem of someone who thinks he hasn't got to try so hard once he's married. I would imagine he was this way during his first marriage too. As he was widowed, presumably his first wife was happy, or at least tolerated his behaviour.
I'll be honest - once a week is about as much sex as I have with my wife at the moment. You say that you have no sex drive. Put the porn to one side for a moment. Are you happy with the amount of sex you are having? If you are, I would suggest that we need to think about the way you react to him watching porn. If you aren't, maybe you would react to the porn differently if you were getting enough sex.
You seem to regard the porn as a competitor to you, almost as if he is being unfaithful, which hurts. You aren't the only woman who thinks like that. I have a good friend who divorced her husband partly because he watched porn.
From a male point of view, it isn't a competitor at all. I don't watch porn and/or masturbate as an alternative to having sex with my wife. It is something I do as well as, not instead of. It certainly doesn't reduce the amount of sex in our relationship. Indeed, it sometimes helps - occasionally we will watch something together and, as my wife also gets turned on by porn, it will usually lead to excellent sex.
For what it is worth, research has established that there is a good biological reason why men masturbate. It gets older sperm out of their system so that, when they have sex with their partner, she gets all the fit, healthy, new sperm.
It is possible your husband is using porn as an alternative to sex if he finds it too difficult to have sex with you, e.g. because he isn't sure how to start (it is surprising how that can happen to a couple over time) or because it takes too long to get you aroused. Your description hints at that with his stuff about going to bed too late, etc. I wonder how he would react if you seduced him on the floor of the kitchen. I'm sure you know this but you don't have to wait until you go to bed.
I think he is probably still loves you and would be devastated if you left. However, he obviously finds it difficult to communicate that to you. I wish I could get into his mind. Why doesn't he want to talk to you about your work? Is it that he feels he can't contribute anything? Is he genuinely not interested? Is it that something you've said or done has wrongly made him feel he can't talk to you about that? Unfortunately I have no way of knowing.
Similarly, why does he not want to take you out more? Does he feel guilty going out without the children? Is he just a homebody who isn't that interested in going out (as opposed to not wanting to go out with you)?
All of this feels like a personal rejection to you but is it? Is he actually rejecting you or is this just the way he is? If I were to ask you to list 20 ways in which he shows you he loves you, could you answer that question?
I don't have any easy answers I'm afraid. What I'm trying to do is challenge your way of thinking about the situation. I don't know if that will help at all. I am, after all, a man! But I can empathise. I felt like this during my first marriage. By the time she finally showed that she did love me it was too late. So I know how you are feeling.