Lenore, who is installing the granite slab shower? Start there and make your priority be hiring a VERY experienced tile professional - one you have vetted carefully to insure that their qualifications meet your design requirements and product selections. Stone slab installations are not for the B team, and especially in a wet area. Can it be done? Yes, depending upon the structure over which a solid surface stone is installed and the methods used. We install solid surface shower walls regularly, but from an empty cavity which we build to meet all specific design and installation specifications, including the pan. A solid stone slab cannot be supported by the pan (it is not meant to support a slab wall). A slab wall is pinned and tied so it is self supported, not supported by a pan. This is a very complex installation. A LOT to consider. As one example, the thickness of your slab? 2cm. 3cm? (effects the weight load on the space). And I could go on. After you have carefully vetted the qualified tile contractor, have this conversation with them. If you question or want to clarify their recommendations, check back or seek professional advice from other qualified sources. Please consult with a professional onsite. Good luck with your project.

The install for tile would require an assessment of the substrate. If this is on plywood/wood joist system, then the "deflection rating" for the joists/subfloor comes into play. That would have to be assessed anyway. Adding a piano on top won't do anything - assuming the deflection rating is good enough for tiles. If this is on cement slab, then there is very little concern about tiles cracking due to weight. Cracking due to improper substrate prep or install is of greater concern. The echo/noise the tiles will create will be ASTONISHING. Playing a piano in an area with tile will be like playing the piano in a bathroom...lots of echo and reverb. I prefer the slightly better acoustics a wood floor can offer. It isn't a huge difference but it can be enough to take the experience out of the "horrible" range and into "not nice but acceptable" category. The area rug will help, but it isn't enough to counter act all of the echo a tiled floor will create. Just for fun, have the carpet removed and then leave the space uncovered for a few days. Go ahead and play the piano in the uncovered space. That's about as good as you will get to understanding the difference the carpet made in that room. Tile will make the acoustics worse. Wood will make them slightly better. Just a thought.

We have 24X24 marble floors throughout our main living area and into kitchen. It wasn't by choice, came with the house when we bought it. I didn't care for the high shine so had them refinished and honed giving them a flat matte finish. They aren't as hard to keep clean as I first feared, but I do spot wipe them often and clean every week or two with a steam mop. You do have to be careful about etching with marble, dripping something like lime juice will ruin that spot on the floor, learned that the hard way with some margaritas : ) We don't have in floor heating, so can't answer to that, but should be fine with both porcelain or marble. The marble floors keep the house cool in the summer. Of course porcelain is much much cheaper. If you find a porcelain that you like you will save a ton of money.

Do a search for porcelain and you will find some threads about breakage. I don't have firsthand experience, but I have read threads about this here on Houzz.