Our first question when we see these sub-standard installs is how did you hire your tile installer? Was a GC involved? So many things wrong about this. You do know that a slope issue relates to what is under the tile? Replacing the tile to solve the slope issue is concerning. Oxidization prior to use? Who bought the tile and from where? The metal trim is an unusual design choice (made by who?) with traditional marble but that being said the tile should abut the metal trim (no visible joint there - that's what the trim is suppose to cover), and the lippage, and, and... Sorry you are dealing with this but it does suggest that qualified labor was not involved. Always vet the contractors involved in your projects and be certain that quality materials are selected and approved by said contractors. We suspect you could have further issues beyond aesthetics down the road. Ask how the installation meets industry standards, including how they prepped (what's under it) the shower. Marble can be a beautiful finish if it's a quality marble and in the hands of a skilled tile installer with verifiable experience, especially in a wet area. https://cdn.ymaws.com/www.tile-assn.com/resource/resmgr/docs/13_questions_to_find_a_tile_.pdf

Our general contractor was involved who insisted that we use this particular subcontractor despite our written hesitation and voiced discomfort.

Could I have someone verify that this cant simply be ”polished out?”

Then it's on the GC. Ask him/her how the tile contractor is qualified as an industry professional who is trained and experienced with standards based tile installations. See the link we attached.

Lippage like that is baked into the (POOR) prep and installation. You can not polish that out without a significant aggressive grit, taking off a large thickness of a tile. That is not an acceptable method of correcting a job with that poor quality materials and labor. Were you also directed to choose the materials from the GC's supplier, or how was that sourced? A marble shower like that is a very high end investment, with many tens of thousands of dollars spent in high quality materials and labor. It is not a 3K builder special. Or, it should not be.

NO the lippage can not be polished out. The marble tile will lose its "factory" shine and it is pretty much impossible to get a new polished finish on a vertical surface with small tiles. On large floor areas they can use heavy floor polishers, but not on vertical walls.

And nothing else can get the same polished finish they get on the original full slabs with specialized equipment at the quarry.

I threw out $8K worth of custom waterjet cut marble backsplash from Artistic Tile when my tile installer thought he could buff down some of the lippage, and this was a good guy who was meticulous and precise. He didn't realize you couldn't get the "factory" shine back. I took the hit on the material, but he covered the cost of a marble refinisher to try to bring the shine back for 2 days and then the labor to remove the marble and install a new tile backsplash.

Here are the photos showing BEFORE and how you get a clear reflection and shine on the tile. You can see all the details of the doors and blinds.

AFTER the tiles were "buffed" and the marble restorer tried to bring back the polished finish.

This might not matter much in a shower that does not get natural light, but was a big issue for this kitchen with lots of natural light. The marble looked dull.