First year having Walking onions, I am in middle Italy that should be 9b or little lower.
They shouldn't have any issue with the hot summer, they are very very rough plants that grow in any conditions. I put even some in some super dry soil where usually weeds appear hoping to fill the place and they are growing, slowly but growing.
Did you plant them too early or too late? I noticed when first transplanted they can be very delicate, got several die due to unknown reasons( maybe cold or too much water?) since I planted them end of winter, so try planting them in a mild weather so that they have time to establish.