Hi, where is the 'dirty' part of the tiles? My suggestion is that the tiles were not actually dirty, but had a variation in the finish across the board that was only noticeable when they were laid. This is a good sign as it means that the tiles have been hand made or at least hand finished. Putting the acid on them will have damaged the surface. I do not recommend sanding as you risk losing the pattern and damaging further.

Ania, can you post photos when it is finished. It will be good to see those colours combined. I also love that inspiration photo and have it saved. I don't know what particular thing makes that bathroom stand out but it is beautiful. I want those exact same colours in my bathroom but a too scared to use timber tiles as I am scared they will date so am bringing in timber via accessories such as picture frames (I think). What is the grey tile in your photo and where is it from...it is very nice.

because the water could seep through the joints between the new tiles and sit between the two layers and affect the seal and also raise the height of the floor and affect the slope to the waste outlets..but, why do you want to retile if you're not using the bathroom??...for immediate resale consider bathroom wall to wall bathroom carpet, new wall paint, new vanity, taps and light fittings which should also be cheaper than tiling over the top and, if it's not allowed by the body corporate, it will be a risk to future sales as it should not satisfy a proper building inspection

I would agree that gloss tiles are not appropriate for wet area floors, and the larger the tile the more chance of slipping. Grout lines seem to provide a bit more traction, so the larger the tile, the fewer grout lines and the less traction. However the more textured a tile, such as in a non-slip tile, the more dirt sticks and the harder it is to get clean. So the best answer for a bathroom floor is a smooth matt, smaller format tile. If you are using a doorless shower recess make sure you allow for an additional floor waste and grading outside the shower recess to minimise water pooling around the floor after you have had a shower. Best of luck, Dr Retro of Dr Retro House Calls