it sounds like they have been through a lot. transplanted, I'm not sure if the mushroom compost was that good for them, cut them back, caterpillars eating them, fertilized and epsom salts, only 4 hrs of sun, when they need at least 6. very stressed. I would refrain from stressing them more.

We've had above average spring rainfall here (way, way above average). Since the roses are planted at the bottom of a slope, I wonder if they're too wet.

The problem might be tied to the grubs you mention.
I found that I lost my pink Knockouts to vole damage. Moles burrow through the ground looking for grubs to eat. They are followed by voles that eat rose roots; I think my voles have a preference for KnockOut roots and although one Knockout has put out roots as the vole was chomping off almost all of the other roots, it will take years for that one to recover.
Good soil attracts moles and voles. Rose roots feed the latter.
I no longer remove rocks from the soil for my roses as I plant them. I figure that roots near the rocks have a chance of survival when the moles come in.

It's hard to tell without looking at your soil, but I am going to make a guess based on my own difficult experience. My garden soil is terrible, a gluey, rockfilled mess that has been amended by myself and the previous owner, but I slowly learned with the overwhelming mess of clay and bottomless number of fill rocks large and small, the amending wasn't nearly enough. I did learn to dig, dig, dig in more organics AND, based on a comment in a rose book, add Perlite.

You see, things would be fine in the cooler spring temps, but when the temps soared, that airless rocky clay just seemed to be choking the life out of the bushes. Plus, I later read that iron moves particularly slowly in heavy clay, and sure enough by late summer some of my bushes began to have typical lack-of-iron symptoms (chlorosis).

My symptoms were poor sad growth, and some bushes would just die. I also had a disgusting grub problem, I would dig up the soil and there was grub after disgusting grub I mashed. Then I treated my garden with milky spore disease. All the amending, perlite, and milky spore disease seems to have improved things, but the improvements continue, and it IS back-breaking work.

So I am wondering if you have lousy soil + you said your garden is at the base of a hill, so maybe you have water pooling there drowning/suffocating the roots. And the problem is really showing up as the temps rise?

Hmmm. I wondered about the mushroom compost too. I spoke with the man at the nursery. He felt that the bagged mushroom compost would be okay for them. He recommended watering around the drip line with a rooting hormone once a week for three weeks. Since we are behind in rain by about 3" already this year and we had a terrible drought last year, I wouldn't have thought that we had over-watered them. As a matter-of-fact, we started watering them because we thought they were looking sad due to lack of moisture. I could go out and dig around and see if it is really wet out there. We do have some mole/vole type rodents someplace because the kitties keep leaving them in the carport (yuck, thanks a lot, cat). Grubs can be a problem too. I know we have them other places in the lawn and other gardens spots.

My sister grips and complains because she has never been successful growing roses here, or azaleas.