Mulch will help keep the soil temps constant and moist which will help keep your cool weather crops alive longer. But mulch will only do so much. Look for varieties that do well in heat for a longer harvest period. you can also shade cool weather plants with snow fencing, shade cloth, tall plants, etc..

Making sure cool weather plants are well water is essential for longevity as well.

Lucy

Being from Michigan also I understand your plight ( 85 degrees one day 45 degrees the next)
I love growing cool weather crops. They are my best crops. I think because they are so simple to grow.

I have found when the weather gets to warm in late may and June make sure you water them frequently to keep the soil moist. Especially beets,radish and greens. Mark