Shame on him. Some rental yards have 4" angle grinders that hold a 4" dry "diamond blade". This tool revolves at 11,000 rpm, so be careful and wear eye protection. It is quick and fast. Cant rent and want to buy?; brand new, $19.95 to $89.95 and may or may not include the blade. Harbor Freight, Northern Tool Supply, Lowe's, the Depot, etc., or look for on line sources. Major mfg., DeWalt, Makita, Bosh. Armen Tavy

I wish I knew what you are talking about, because I did not know there was "a study" on either subject. I do know that in "Foreign" lands, tile is a mainstay that becomes part of the home and is rarely changed. Localy (in America) "we tend to get tired of the old" and "want the new". Now your feasibility question I can understand. Tile is stronger, cleaner, and easier to maintain when compared to any other kind of floor covering and that conclusion does not need "a study".  Armen Tavy

You did not say why you are removing the old tiles. Was there a problem or did you just want a change? If you are careful removing all the old tiles, you can install new tiles over the old DuRock. However, if the previous installer did a proper installation, salvaging the underlayment will literally be impossible. If the tiles come up easy without ruining the DuRock, then you will benefit from the previous installers shortcomings. If you want your new installation to last, please follow industry guidlines when re-tiling. If you want to know what they are, just ask.Armen Tavy

When I hear of poor workmanship it is a disappointment to me as well, and there is no excuse for it if you ask for and then check 3 references, as you should. It is difficult to access how difficult it will be to remove the tiles. Much depends on the whether the person with poor workmanship also is not proficient in getting a good bond to substrates or the back side of tiles. The tiles may come up easy but the mortar may not, or you may be lucky, well not really, and the mortar may come up with the tiles. In any case, I would assume that the same installer would not do the next installation.

There are certain tools that make this job easier and the main one I recommend, if a hammer and chisel does not work well, is the “Makita HK1810” Power Scrapper. Cost varies with the retailer from $360 to $400 and worth every dollar in the time and effort it can save.  If the installer is willing to remove his inept work, then it is his problem. If this same installer waterproofed your shower pan, I would make sure the weep holes are functioning and conduct a “leak test” before retiling. Need more help, just write again. Armen Tavy

You can tile over tile, with the right products, if you have the room. How you will terminate the new tiles so the old ones cannot be seen at the perimeter edges of the installation, must also be considered. A test patch of "Liquid Latex Modified Mortars" would have to be done with an overnight application. If it is very hard to remove (scrape) the next day, then you can tile over tile with this product, or you can use a product like TAVY "Thin-Skin" Tile Underlayment to assure a proper bond without an overnight test. The 2" tiles on the floor of the shower are best tiled over with the Liquid Latex Modified Mortar (test first). The drain cover and screen must be raised with a drain extender called "Extend-O-Drain" (look for both items on line). Measure the diameter of your drain screen cover (center to center of the 2 screws) and order the appropriate size. Remove all evidence of “soap scum”. Armen Tavy

Kitchen n do not require a lot of grout and some installers refuse to understand the importance of pre-mixing the dry contents of a grout container before using part of it for these smaller installation. If this is not the cause then it could be that the installer diluted the grout color with to much water in the mix as well as too much water in the grout "washing". If the color of the grout in the "wet stage" was initially darker, when applied "wet", and then dried "whiter", the above scenarios are more suspect. The age of the grout could also have a bearing on your disappointment of the final shade. BUY brand new grout, mix the dry contents thoroughly before adding water and try one last time. Do not make the grout "soupy" (make it more like mayonnaise) or use too much water in the sponge when washing. Armen Tavy

If the plywood is Douglas Fir, then a "Liquid Latex Modified Mortar" will work, if not, you could use cement backerboards or tile underlayment membranes like, Schluter Ditra Mat, Protect Wrap, TAVY "Thin-Skin", or most other name brands. Travertine Tiles need an experienced Travertine Installer, with a good reputation. Armen Tavy