I believe family and in-laws are God's plan to make us understand unconditional love. Anyway, you sound like you really have two problems: your husband's attitude when Mommy Dearest appears and your overall relationship with your husband.
Problem #1: MIL and DH. You cannot change people nor raise adults. You can only control your own behavior and train children. Either train the children to prepare for grandma's visit by living by the rules or have constant strife with Dad. Consider the once every two years visit as a sacrifice your family makes in order to keep peace within the extended family. B. Talk to your husband, understanding that he might not see his behavior as odd and be unwilling to change. Again, you cannot control or fix people, but you can offer clear examples of the inconsistencies and help him understand that his conduct might affect his relationship with his children as well as their relationship with their grandmother. He probably does not want them to dread grandma's biannual visit. Then, I'd come up with activities that make for happy family memories. For instance, invite Grandma to take the kids to the park. They'll be too tired when they come home to make a mess.
As for the world revolving around the DH, I think that's just marriage. But, I do appreciate the fact that the men can teach us about not losing ourselves as wives and mothers. There should be a balance. You're not overreacting to the weekly darts game. On the surface, there's nothing wrong in his playing darts. But, weekly? At a bar? He's not using sound judgement. He's got too much work to do at home to do that weekly, anyway. And, what about you? If he's out once a week, you should get "me" time, too. Not possible? Then, how about his giving you a few hours on Sat am or Sun. pm? We're seeking balance, here. Your job is 24-7 and you need a break. As for the trust factor, that is earned, not blindly given. You cannot control another person's behavior, nor do you owe anyone blind trust if they've shown that they have poor judgement. But dh is an adult, not a child. It's not your job to police him nor raise him. Nor should he live in a bubble because he's made mistakes. Either your relationship will grow or it will deteriorate in deceit. That will be his choice. Meanwhile, you do your part and find an outlet other than the children and him to explore that side of you that is human. It can even be activities you do with the children, like painting or reading or woodworking, etc.
Marriage counselling is there when BOTH parties believe they need help. You might want to consider therapy for yourself if it will help you process some things. I know that prayer, having a good support system, seeing veteran married people who enjoy each other, and learning what does not work have helped sustain my own life as a wife and mother. Make sure you're getting good advice and that you are very in tune with who you are.