A tile 3/8" 18" wide and 36" Long weighs about 20 pounds. Two people could stand shoulder to shoulder in that space. I would think two people would average out to about 350 pounds. What happens when you fill a glass of water and place it in the bathroom on the floor. Can you walk around the room without spilling any water? Water should be to the top. Can you dance around the room?

I have SW ivory lace trim in our mbath... I chose it because it looked nice with the tile we have, but it looks fine with our white kohler sinks and the toilet. it is a warm white without being too yellow. our cabinets are a med-dark brown and the tile has some grays/greige and white colors in it.

People talking about waterproofing that have never tiled anything. By almost every code in every state a simple plastic sheet or roofing felt behind cement board is sufficient and in some cases preferred. People hear a little information and just keep repeating it. Roll on and membrane systems were developed for high rise and steam showers set ups. Sometimes depending on the tile material allowing the wall to breathe is actually better. Not to say there isn't plenty wrong here. First off that size marble is a disaster for everyday shower use. It should be a mud job those are very heavy panels. The seams should be taped. I get the edges overlap the wet area and will be covered by a door but that is where there is a waterproofing issue. Membrane waterproofing systems are made to go over sheetrock which was the problem to begin with. In the 90's green board was touted as being waterproof which of course it wasn't. That is the entire reason for all these bathroom failures. Cement board isn't waterproof but it also will not fail if wet so you can let it get wet all day long and nothing will happen. In STANDARD shower set up in a house simple is better. Membrane set ups are not easy to use. Mfg's don't make the required thin sets to install and a lot of mixing and matching is used. Remember all those green board showers looked great till 15-20years later when they were ripped out and everybody saw all the mold and crumbling sheetrock. STOP YELLING WATERPROOFING AND READ THE PRODUCTS ACTUAL TECH SHEETS. NOWHERE ON CEMENT BOARD DOES IT SAY YOU NEED A ROLL ON OR MEMBRANE WATERPROOFING SYSTEM. I would rather an uneducated installer use a simple system then use a system they don't know and screw it up and cause more issues than are corrected.

From here it looks like the shower was not properly waterproofed and the pony wall is swelling due to the wood getting wet. Testing for moisture levels outside the shower may help, right before you remove the drywall from the outside of the pony wall to check for waterproofing (if any) and leaking.