To my knowlege, the uterus in goats rides in front of the intestines, so strangulation wouldn't be an issue. I think the worst that could happen (I'm just thinking hernias in general) is the hernia would enlarge during the pregnancy and be a problem for her later.
My daughter developed an umbilical hernia when she was a month or so old, and I thought I needed a "belly band" for her, but it turned out to be an old fashioned remedy NOT recommended. It went back in when she was a year old on its own, but it made me think about your goat . . . I wonder if a kind of abdominal truss -- not tight, but just support, like those lower back support things we humans wear -- fasten with velcro, and viola, you have support over the stressed area which would distribute the stress and not focus it where she is already weak.
I'm not a goat expert at all, so with a grain or two of salt, here.
Another thing about umbilical hernias . . . just the way the abdominal muscles are, it doesn't seem possible for it to "spread". Now an abdominal hernia is different, where the two sides of the "six pack" separate in humans due to multiple pregnancy or obesity. On a goat, I can understand your vet's reluctance to repair an abdominal hernia, but umbilical repair wouldn't "spread" as it would have nowhere to spread to, it would just re-open after a time, but in the same place. Perhaps a second opinion if you don't already have one?
Reread paragraph #3

. I've read this thread a couple of times and didn't respond, but impulse won out today.