Hydrangeas are like magic. If you're patient, they'll adapt to just about anything - unless it's just hellish where they are but there are other things you can do. If the sun comes up behind the house, it might just be a matter of moving it out a little further away from it so that it gets some a little bit earlier, before it gets to the beating down stage, so it's not as much of a shock. Or you could add something interesting that would give it a little bit of relief. It doesn't need to be completely shaded. It could even be a flush of shade that simply moves across it during that hottest part of the day.
The sun moves pretty quick - not a fast as the moon, but it moves constantly so even a few tall stalks of another plant could give it some relief during that part of the day. Or maybe just an olla.
You don't want to water it or have it wet with the sun glaring on it as the moisture acts like a bunch of tiny little magnifying glasses but an olla or one of those 2 liter pop bottle things, with some small holes buried near it (I forget what they call them) or something that provides water under the top of the soil where the roots can easily get at it.
Yours don't look too bad and, as you say, they recover pretty quickly once the sun's passed over the high spot a ways, right? Just that little extra water, closer to home, would release some of the energy it uses to increase the reach of its roots in search of water (Which SPENDS some of its water) and have more energy to move the moisture up the stalks and to the flower heads. It's a bit like cutting leaves when propagating from cuttings to release some energy from photosynthesis so your cuttings can add that energy to root production.
It looks like plants just sit there and don't do much but they're very busy little multi-tasking dudes and, like people, they only have so much fuel and muscle to work with. So if you can free up some fuel and strength from its other efforts, it can redirect that energy into protecting itself AND into learning about its new home and adapting to its conditions.
And you probably already know all of this - I'm just rambling. Sorry.
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