Chris,
After your great post, I think us experts must mostly have decided you don't need any help. Your post is full of concern, sensitivity, and compromise, and that's all you need to help your partner. But you asked for help, so here are a few tips from someone who's had support from their partner for 35+ years...
I think your suggestion of entering her blood results is great. I use a really old, clunky system (Diabetes Mentor) and it really helps when my Other Half enters the results. Mr Happy is right though that most systems these days accept results automatically, so maybe you can help her find the right system for her, by piloting a few. I think it's more important than it might seem.
Without a set of charts that really communicate well what's going on, she might switch off. And it is back to "manage your blood sugars into the right range for ever, or something bad will happen". With a good chart then the task becomes simpler - "do you think you can do something to get of that peak that always happens just before lunch?". That makes things easier, and seeing the improvement in the charts gives really positive feedback. (I wouldn't recommend Diabetes Mentor as its so old, but it has a great chart called the standard day. It takes all the results over whatever period you select and plots them, and works out what the day looks like. You say her results are erratic - a chart like this could help spot the underlying pattern.)
As for meals, I agree it's good if you can both eat the same healthy food. We take turn about in making our evening meals, each one choosing what to make. The idea was that my OH wouldn't have to eat healthily every day. Of course he generally makes healthy food, but it's also clear he things anything can be made healthy with a portion of salad on the side
The other thing about food is that, as a man, you should eat more calories than your partner. This is fraught with danger, if she is being disciplined about what she eats. If we share a healthy meal, then my OH is soon tucking into a sugary or fatty snack it can be really irritating. So, a bit of sensitivity with your extra calories will help. Or a large wardrobe you can eat a Dairy Milk in.
I wish both of you all the best. I am sure with your support the diabetes will be less daunting.
Christine