I'm very sorry to read about the puppy. It happens with large litters and bigger breeds. My whelping box fits next to my bed (the joys of a smaller breed), and I'm easily woken by squealing and squished puppies . I got told once, how true it is I don't know, but it makes sense to me, animals have litters or multiple babies because deaths are all part of it. It's modern medicine and constant supervision that prevents as many deaths as nature would sometimes dish out.
Even the best of dams lay on pups.
I wouldn't worry too much about the hard tiles, if it's really hot I'd prefer the cool of the tiles to be able to come thru the blanket but of course the harder the surface the more easily she squishes them. But it's one of those things that you just have to look at and weigh up sometimes on a day by day basis in horrible weather.
I' always a zombie for the first couple of weeks of a litter.