I'm unsure about some of this. Did he want you to walk away and leave her on the corner? That is definately not safe. One of the rules for my grandchildren has always been that they stay with the adult. In this case your youngest has not only not stuck to her agreement but she's broken the rule to stay with you. Both are lessons to be learned.
What is your usual discipline method for these sort of things? If you and your husband has talked out and agreed upon rules and discipline measures you'll be more likely to have less disagreements of this sort.
I do wonder why your husband couldn't be more flexible and hold one girl's hand or carry packages so that you could hold both girls' hands. It is often helpful to renegotiate when the the first plan doesn't work. The goal is to teach children how to get along with others and there is more than one way to do almost everything. We don't have to stick to the first plan. I think to alter the plan is important when children are not feeling well.
For example you could talk about the first plan and see if it'll work. When it doesn't you can suggest that you come to some other compromise while acknowledging that sometimes we agree to a plan and then later find that it isn't working for us.
It was easier for the oldest daughter to stick with the plan because she started the walk out rested knowing that she would get to hold your hand later. What incentive does the youngest one have to let go of your hand?
It might help to have a plan to do something once you got home. For example you could say to the youngest one. I know that you're tired and don't feel well. I'll hold you when we get home. I see holding hands and holding of any sort as a nurturing act. Your youngest was telling you she needed more personal attention from you.
How does your husband relate to the girls? Does he nurture them? Play with them? Take them to the park? Hold their hands. Cuddle with them while watching TV? At 50 he may not know how to do that. If so would he be willing to take a parenting class with you and/or read some parenting books?
I'm suggesting that it would help if you and your husband had some counseling together. I know I'd be very hurt and angry if my husband stopped working. I think it's possible that he's critical of you because of his own negative sense of self. When you tell the other person what they're doing wrong you cause them to be defensive and thus keep attention away from what you're doing wrong that could be changed. This creates a hostile environment in which it's difficult to come to agreement about parenting.
I was a first time single mom of a 7yo when I was 42. I entered into parenthood feeling that even tho I'd not parented before I knew a lot of concepts that I could put into action. I also had to follow CSD rules which were at odds with some of my values. I soon lost confidence. Counseling helped me a great deal to realize that I could parent and still be a confident person even when my parenting was not effective.
Talking to others helped me to keep a more positive perspective on what I was doing. I think that writing to us on MamaSource can provide some of that for you.
Truly, parenting is the most difficult job available. I was a police officer when I started parenting and the parenting was much more difficult than the policing. As a police officer I had definate laws, departmental rules, and many people with whom to problem solve. As a parent I felt somewhat isolated from other parents. Friends my age were nearly thru with raising children. And there are no definite rules to follow. Parenting is a matter of trial and error. Parenting books and classes help a lot but it felt like there was too much information and how do I know what advice to follow? So I tried what seemed to fit with my value system and my daughter's personality. Sometimes it worked. Sometimes it didn't.
So.....it's very important for you and your husband to "be on the same page" about how you want to parent. It's very important that each supports the other in front of the kids. And disagreements need to be handled out of their hearing.
Find times for the two of you to talk about the things that you disagree about and work on finding a middle ground that you can both agree with. This might work better if you do it with a 3rd person.